INTERSTATE POWER AND LIGHT COMPANY (an Iowa corporation) 5.50% SENIOR DEBENTURES DUE 2025 PURCHASE AGREEMENT Dated: July 18, 2005

EXHIBIT 1

INTERSTATE POWER AND LIGHT COMPANY

(an Iowa corporation)

5.50% SENIOR DEBENTURES DUE 2025

PURCHASE AGREEMENT

Dated: July 18, 2005

TABLE OF CONTENTS

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SCHEDULES

Schedule A — List of Underwriters’ Purchase Amounts
Schedule B — Pricing Information

EXHIBITS

Exhibit A — Form of Opinion of Company’s Counsel
Exhibit B — Form of Opinion of Company’s In-House Counsel

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INTERSTATE POWER AND LIGHT COMPANY
(an Iowa corporation)

5.50% SENIOR DEBENTURES DUE 2025

PURCHASE AGREEMENT

July 18, 2005

Barclays Capital Inc.
Banc of America Securities LLC
Lazard Capital Markets LLC
c/o Barclays Capital Inc.
▇▇▇ ▇▇▇▇ ▇▇▇▇▇▇
▇▇▇ ▇▇▇▇, ▇▇▇ ▇▇▇▇ ▇▇▇▇▇

Ladies and Gentlemen:

     Interstate Power and Light Company, an Iowa corporation (the “Company”), confirms its agreement with Barclays Capital Inc. (“Barclays Capital”), Banc of America Securities LLC (“Banc of America”) and each of the other underwriters named in Schedule A hereto (collectively, the “Underwriters”, which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), for whom Barclays Capital and Banc of America are acting as representatives (in such capacity, the “Representatives”), with respect to the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective principal amounts set forth on Schedule A of $50,000,000 aggregate principal amount of the Company’s 5.50% Senior Debentures due 2025 (the “Securities”). The Securities will be issued pursuant to an indenture dated as of August 20, 2003 (the “Indenture”) between the Company and ▇.▇. ▇▇▇▇▇▇ Trust Company, National Association, as successor in interest to Bank One Trust Company, National Association, as trustee (the “Trustee”). The term “Indenture,” as used herein, includes the Officer’s Certificate (as defined in the Indenture) to be executed in connection with the offering of the Securities, establishing the form and terms of the Securities pursuant to Section 301 of the Indenture. The Securities are to be issued in book-entry form and will be issued to Cede & Co. as nominee of The Depository Trust Company (“DTC”) pursuant to the blanket letter agreement, dated October 1, 2003 (the “DTC Agreement”), between the Company and DTC.

     The Company understands that the Underwriters propose to make a public offering of the Securities as soon as the Representatives deem advisable after this Agreement has been executed and delivered.

     The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement on Form S-3 (No. 333-114065), for the registration of the Securities under the Securities Act of 1933, as amended (the “1933 Act”) and the offering

thereof from time to time in accordance with Rule 415 of the rules and regulations of the Commission under the 1933 Act (the “1933 Act Regulations”). Such registration statement has been declared effective by the Commission, and the Company has filed such post-effective amendments thereto (or such preliminary prospectus or preliminary supplemental prospectus) as may be required under the 1933 Act and the 1933 Act Regulations, and each such post-effective amendment has been declared effective by the Commission. Such registration statement is referred to herein as the “Registration Statement;” and the final prospectus and the final prospectus supplement relating to the offering of the Securities, in the forms first furnished to the Representatives by the Company for use in connection with the offering of the Securities, are collectively referred to herein as the “Prospectus;” provided, however, that all references to the “Registration Statement” and the “Prospectus” shall also be deemed to include all documents incorporated therein by reference pursuant to the Securities Exchange Act of 1934, as amended (the “1934 Act”), filed prior to the applicable date; provided, further, that if the Company files a registration statement with the Commission pursuant to Rule 462(b) of the 1933 Act Regulations (the “Rule 462(b) Registration Statement”), then all references to “Registration Statement” shall also be deemed to include the Rule 462(b) Registration Statement. A “preliminary prospectus” shall be deemed to refer to (i) any prospectus used before the Registration Statement became effective and (ii) any prospectus or prospectus supplement that omitted the information to be included upon pricing in a form of prospectus or prospectus supplement filed with the Commission pursuant to Rule 424(b) of the 1933 Act Regulations and was used after such effectiveness and prior to the initial delivery of the Prospectus to the Representatives by the Company. For purposes of this Agreement, all references to the Registration Statement, Prospectus or preliminary prospectus or to any amendment or supplement to any of the foregoing shall be deemed to include any copy filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system (“▇▇▇▇▇”).

     All references in this Agreement to financial statements and schedules and other information which are “contained,” “included” or “stated” (or other references of like import) in the Registration Statement, Prospectus or preliminary prospectus shall be deemed to mean and include all such financial statements and schedules and other information which are incorporated by reference, as of such applicable date, in the Registration Statement, Prospectus or preliminary prospectus, as the case may be; and all references in this Agreement to the Registration Statement, amendments or supplements to the Registration Statement, Prospectus or preliminary prospectus shall be deemed to include the filing of any document under the 1934 Act which is incorporated by reference, as of such applicable date, in the Registration Statement, Prospectus or preliminary prospectus, as the case may be.

     SECTION 1. Representations and Warranties.

     (i) Compliance with Registration Requirements. The Company meets the requirements for use of Form S-3 under the 1933 Act. Each of the Registration Statement and any Rule 462(b) Registration Statement has become effective under the 1933 Act and no stop order suspending the effectiveness of the Registration Statement or any Rule 462(b)

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Registration Statement has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or, to the knowledge of the Company, are contemplated by the Commission, and any request on the part of the Commission for additional information has been complied with.

     At the respective times the Registration Statement, any Rule 462(b) Registration Statement and any post-effective amendments thereto became effective and at the Closing Time, the Registration Statement, the Rule 462(b) Registration Statement and any amendments and supplements thereto complied or will comply, as the case may be, in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations and the Trust Indenture Act of 1939, as amended (the “1939 Act”) and the rules and regulations of the Commission under the 1939 Act (the “1939 Act Regulations”), and did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading and the Indenture complied and will comply in all material respects with the requirements of the 1939 Act. Neither the Prospectus nor any amendments or supplements thereto, at the time the Prospectus or any such amendment or supplement was issued and at the Closing Time, included or will include an untrue statement of a material fact or omitted or will omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The representations and warranties in this subsection shall not apply to statements in or omissions from the Registration Statement or Prospectus made in reliance upon and in conformity with information furnished to the Company in writing by any Underwriter through the Representatives expressly for use in the Registration Statement (or any amendment thereto) or Prospectus (or any amendment thereto).

     Each preliminary prospectus and the prospectus filed as part of the Registration Statement as originally filed or as part of any amendment thereto, or filed pursuant to Rule 424 under the 1933 Act, complied when so filed in all material respects with the 1933 Act Regulations and each preliminary prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to ▇▇▇▇▇, except to the extent permitted by Regulation S-T.

     (ii) Incorporated Documents.

     (1) The documents incorporated or deemed to be incorporated by reference in the Registration Statement and the Prospectus, at the time they were or hereafter are filed with the Commission, complied or will comply, as the case may be, in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission thereunder (the “1934 Act Regulations”), as applicable, and, when read together with the other information in the Prospectus, at the time the Registration Statement became effective, as of the date of the Prospectus and at the Closing Time, did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading.

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     (2) The description of regulatory matters to which the Company is subject, as disclosed in the Company’s filings with the Commission under the 1934 Act and the 1934 Act Regulations and as incorporated by reference into the Registration Statement, is true and correct in all material respects, except to the extent such description in any specific filing has been superseded, updated or supplemented by such description in a subsequent filing under the 1934 Act or the 1934 Act Regulations made prior to the date hereof or by such description in the Prospectus.

     (iii) Independent Accountants. The accountants who certified the financial statements and supporting schedules included in the Registration Statement are independent public accountants with respect to the Company and its subsidiaries within the meaning of Regulation S-X under the 1933 Act.

     (iv) Financial Statements. The financial statements included in the Registration Statement and the Prospectus, together with the related schedules and notes, present fairly in all material respects the financial position of the Company and its consolidated subsidiaries at the dates indicated and the statement of income, changes in common equity and cash flows of the Company and its consolidated subsidiaries for the periods specified; said financial statements have been prepared in conformity with generally accepted accounting principles (“GAAP”) applied on a consistent basis throughout the periods involved. The supporting schedules, if any, included in the Registration Statement present fairly in all material respects in accordance with GAAP the information required to be stated therein. The selected financial data and the summary financial information included in the Prospectus present fairly in all material respects the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement.

     (v) No Material Adverse Change in Business. Since the respective dates as of which information is given in the Registration Statement and the Prospectus, except as otherwise stated therein, (A) there has been no material adverse change in the condition, financial or otherwise, in the earnings or business affairs of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business nor has there been any developments involving a prospective material adverse change of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business (a “Material Adverse Effect”), (B) there have been no transactions entered into by the Company or any of its subsidiaries, other than those in the ordinary course of business, which are material with respect to the Company and its subsidiaries, and (C) except for regular dividends on the common stock, par value $2.50 per share, of the Company, the 8.375% Series B Cumulative Preferred Stock, $0.01 par value per share, of the Company and the 7.10% Series C Cumulative Preferred Stock, $0.01 par value per share, of the Company in amounts per share that are consistent with past practice, the terms of the 8.375% Series B Cumulative Preferred Stock or the terms of the 7.10% Series C Cumulative Preferred Stock, as the case may be, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

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     (vi) Good Standing of the Company. The Company has been duly organized and is validly existing as a corporation under the laws of the State of Iowa and has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under this Agreement; and the Company is duly qualified as a foreign corporation to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

     (vii) No Significant Subsidiaries. The Company has no “significant subsidiary” as defined in Rule 1-02 of Regulation S-X.

     (viii) Capitalization. The authorized, issued and outstanding capital stock of the Company as of the date hereof is set forth in the Prospectus in the column entitled “Actual” under the caption “Capitalization” (except for subsequent issuances, if any, pursuant to this Agreement, pursuant to existing reservations, agreements or employee benefit plans, or pursuant to the exercise of convertible securities or options outstanding on the date hereof or pursuant to any dividend reinvestment plan). All of the issued and outstanding shares of capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable and none of the outstanding shares of capital stock of the Company was issued in violation of the preemptive or other similar rights of any securityholder of the Company. As of the date hereof, except for 6,000,000 shares of the Company’s 8.375% Series B Cumulative Preferred Stock and 1,600,000 shares of the Company’s 7.10% Series C Cumulative Preferred Stock, all of the issued and outstanding shares of the capital stock of the Company are owned by Alliant Energy Corporation, a Wisconsin corporation (the “Parent”), free and clear of all liens, encumbrances, equities or claims. Immediately prior to the Closing Time, except for 6,000,000 shares of the capital stock of the Company’s 8.375% Series B Cumulative Preferred Stock and 1,600,000 shares of the Company’s 7.10% Series C Cumulative Preferred Stock, all of the issued and outstanding shares of capital stock of the Company will be owned directly by Parent, free and clear of all liens, encumbrances, equities or claims. The Parent is a “holding company” and the Company is a “subsidiary” of a “holding company” as such terms are defined under the Public Utility Holding Company Act of 1935, as amended (the “1935 Act”).

     (ix) Authorization of Agreement. The Company has all requisite corporate power and authority to execute and deliver this Agreement and to perform its obligations hereunder. This Agreement has been duly authorized, executed and delivered by the Company.

     (x) Authorization of the Indenture. The Indenture has been duly authorized, executed and delivered by the Company and duly qualified under the 1939 Act and constitutes a valid and binding agreement of the Company enforceable against the Company in accordance with its terms, except (A) as enforcement thereof may be limited by bankruptcy, insolvency (including without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting the enforcement of creditors’

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rights generally and (B) as enforcement thereof is subject to general principals of equity (regardless of whether enforcement is considered in a proceeding in equity or at law).

     (xi) Authorization of the Securities. The Securities have been duly authorized and, at the Closing Time, will have been duly executed by the Company and, when authenticated, issued and delivered in the manner provided for in the Indenture and delivered against payment of the purchase price therefor provided for in this Agreement, will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms, except as the enforcement thereof may be limited by bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting enforcement of creditors’ rights generally and except as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law) and will be in the form contemplated by, and entitled to the benefits of, the Indenture.

     (xii) Description of the Securities and the Indenture. The statements relating to the Securities and the Indenture contained in the Prospectus conform, respectively, in all material respects to the terms of the Securities and the Indenture.

     (xiii) Absence of Defaults and Conflicts. Neither the Company nor any of its subsidiaries is in violation of its charter or by-laws or in default in the performance or observance of any obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, deed of trust, loan or credit agreement, note, lease or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which it or any of them may be bound, or to which any of the property or assets of the Company or any of its subsidiaries is subject (collectively, “Agreements and Instruments”) except for such violations or defaults that would not result in a Material Adverse Effect; and the execution, delivery and performance of this Agreement, the Indenture and the Securities, the consummation of the transactions contemplated herein and in the Registration Statement (including the issuance and sale of the Securities and the use of the proceeds from the sale of the Securities as described in the Prospectus under the caption “Use of Proceeds”) and compliance by the Company with its obligations hereunder and under the Indenture and the Securities have been duly authorized by all necessary corporate action and do not and will not, whether with or without the giving of notice or passage of time or both, conflict with or constitute a breach of, or default or a Repayment Event (as defined below) under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to, the Agreements and Instruments except for such conflicts, breaches or defaults or liens, charges or encumbrances that, singly or in the aggregate, would not result in a Material Adverse Effect, nor will such action result in any violation of the provisions of (A) the charter or by-laws of the Company or any of its subsidiaries (except for such conflicts, breaches, defaults, events or liens, charges or encumbrances that would not result in a Material Adverse Effect) or (B) any applicable law, statute, rule, regulation, judgment, order, writ or decree of any government, government instrumentality or court, domestic or foreign, having jurisdiction over the Company or any of its subsidiaries or any of their assets, properties or operations, except for any such violations with respect to this clause (B) as would not, individually or in the aggregate, result in a Material Adverse Effect. As used herein, a “Repayment Event” means any event

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or condition which gives the holder of any note, debenture or other evidence of indebtedness (or any person acting on such holder’s behalf) the right to require the repurchase, redemption or repayment of all or a portion of such indebtedness by the Company or any of its subsidiaries.

     (xiv) Absence of Labor Disputes. No labor dispute with the employees of the Company or any of its subsidiaries exists or, to the knowledge of the Company, is imminent, and the Company is not aware of any existing or imminent labor disturbance by the employees of any of its or any of its subsidiaries’ respective principal suppliers, manufacturers, customers or contractors, which, in either case, may reasonably be expected to result in a Material Adverse Effect.

     (xv) Absence of Proceedings. Except as disclosed in the Prospectus, there is no action, suit, proceeding, inquiry or investigation before or brought by any court or governmental agency or body, domestic or foreign, now pending, or, to the knowledge of the Company, threatened, against or affecting the Company or any of its subsidiaries which might reasonably be expected to result in a Material Adverse Effect, or which might reasonably be expected to materially and adversely affect (A) the properties or assets of the Company and its subsidiaries or (B) the consummation of the transactions contemplated by this Agreement or the performance by the Company of its obligations hereunder. The aggregate of all pending legal or governmental proceedings to which the Company or any of its subsidiaries is a party or of which any of their respective property or assets is the subject which are not described in the Prospectus, including ordinary routine litigation incidental to the business, could not reasonably be expected to result in a Material Adverse Effect.

     (xvi) Accuracy of Exhibits. There are no contracts or documents which are required to be described in the Registration Statement, the Prospectus or the documents incorporated by reference therein or to be filed as exhibits thereto which have not been so described and filed as required except that this Agreement and an opinion of ▇▇▇▇▇ & ▇▇▇▇▇▇▇ LLP with respect to the validity of the Securities will be filed as exhibits to a Current Report on Form 8-K which shall be filed with the Commission in accordance with the 1934 Act and the 1934 Act Regulations prior to the filing of the Prospectus.

     (xvii) Absence of Further Requirements. No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any court or governmental authority or agency is necessary or required for the performance by the Company of its obligations hereunder, in connection with the offering, issuance or sale of the Securities hereunder, the consummation of the transactions contemplated by this Agreement or for the due execution, delivery and performance of the Indenture by the Company, except (A) such as have been already obtained, (B) such as may be required under the 1933 Act or the 1933 Act Regulations or state securities laws, and (C) such as may be required by the 1935 Act, solely with respect to filings required to be made with the Commission subsequent to the Closing Time (such 1935 Act filings to be made by the Company).

     (xviii) Possession of Licenses and Permits. The Company and its subsidiaries possess such permits, licenses, approvals, consents and other authorizations (collectively,

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     (xix) Title to Property. The Company and its subsidiaries have good and marketable title to all real property owned by the Company and its subsidiaries and good title to all other properties owned by them, in each case, free and clear of all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances of any kind except such as (A) are described in the Prospectus or (B) do not, singly or in the aggregate, materially affect the value of such property and do not interfere with the use made and proposed to be made of such property by the Company or any of its subsidiaries; and all of the leases and subleases material to the business of the Company and its subsidiaries, considered as one enterprise, and under which the Company or any of its subsidiaries holds properties described in the Prospectus, are in full force and effect, and neither the Company nor any of its subsidiaries has any notice of any material claim of any sort that has been asserted by anyone adverse to the rights of the Company or any of its subsidiaries under any of the leases or subleases mentioned above, or affecting or questioning the rights of the Company or any of its subsidiaries to the continued possession of the leased or subleased premises under any such lease or sublease, except where such would not have a Material Adverse Effect.

     (xx) Investment Company Act. The Company is not, and upon the issuance and sale of the Securities as herein contemplated and the application of the net proceeds therefrom as described in the Prospectus will not be, an “investment company” or an entity “controlled” by an “investment company” as such terms are defined in the Investment Company Act of 1940, as amended (the “1940 Act”).

     (xxi) Environmental Laws. Except as described in the Registration Statement and except as would not, singly or in the aggregate, result in a Material Adverse Effect, (A) neither the Company nor any of its subsidiaries is in violation of any federal, state, local

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or foreign statute, law, rule, regulation, ordinance, code, policy or rule of common law or any judicial or administrative interpretation thereof, including any judicial or administrative order, consent, decree or judgment, relating to pollution or protection of human health, the environment (including, without limitation, ambient air, surface water, groundwater, land surface or subsurface strata) or wildlife, including, without limitation, laws and regulations relating to the release or threatened release of chemicals, pollutants, contaminants, wastes, toxic substances, hazardous substances, petroleum or petroleum products (collectively, “Hazardous Materials”) or to the manufacture, processing, distribution, use, treatment, storage, disposal, transport or handling of Hazardous Materials (collectively, “Environmental Laws”), (B) the Company and its subsidiaries have all permits, authorizations and approvals required under any applicable Environmental Laws and are each in compliance with their requirements, (C) there are no pending or, to the knowledge of the Company, threatened administrative, regulatory or judicial actions, suits, demands, demand letters, claims, liens, notices of noncompliance or violation, investigation or proceedings relating to any Environmental Law against the Company or any of its subsidiaries and (D) there are no events or circumstances that might reasonably be expected to form the basis of an order for clean-up or remediation, or an action, suit or proceeding by any private party or governmental body or agency, against or affecting the Company or any of its subsidiaries relating to Hazardous Materials or any Environmental Laws.

       Payment shall be made to the Company by wire transfer of immediately available funds to a bank account designated by the Company, against delivery to the Representatives for the

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respective accounts of the Underwriters of certificates for the Securities to be purchased by them. It is understood that each Underwriter has authorized the Representatives, for its account, to accept delivery of, receipt for, and make payment of the purchase price for, the Securities that it has agreed to purchase. Either Barclays Capital or Banc of America, individually and not as a representative of the Underwriters, may (but shall not be obligated to) make payment of the purchase price for all or a portion of the Securities to be purchased by any Underwriter whose funds have not been received by the Closing Time, but such payment shall not relieve such Underwriter from its obligations hereunder.

       SECTION 3. Covenants of the Company. The Company covenants with each Underwriter as follows:

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         SECTION 4. Payment of Expenses.

         SECTION 5. Conditions of Underwriters’ Obligations. The obligations of the several Underwriters hereunder are subject to the accuracy in all material respects of the representations and warranties of the Company contained in Section 1 hereof or in certificates of any officer of the Company or any subsidiary of the Company delivered pursuant to the provisions hereof, to

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the performance in all material respects by the Company of its covenants and other obligations hereunder, and to the following further conditions:

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     (i) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, arising out of any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement (or any amendment thereto), or the omission or alleged omission therefrom of a material fact required to be stated therein or necessary to make the statements therein not misleading or arising out of any untrue statement or alleged untrue statement of a material fact included in any preliminary prospectus or the Prospectus (or any amendment or supplement thereto), or the omission or alleged omission therefrom of a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading;

     (ii) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, to the extent of the aggregate amount paid in settlement of any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or of any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission; provided that (subject to Section 6(d) below) any such settlement is effected with the written consent of the Company; and

     (iii) against any and all expense whatsoever, as incurred (including the fees and disbursements of counsel chosen by the Representatives), reasonably incurred in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission, to the extent that any such expense is not paid under (i) or (ii) above;

provided, however, that this indemnity agreement shall not apply to any loss, liability, claim, damage or expense to the extent arising out of any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives expressly for use in the Registration Statement (or any amendment thereto) or any preliminary prospectus or the Prospectus (or any amendment or supplement thereto); and provided, further that this indemnity

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agreement shall not inure to the benefit of any Underwriter or any person who controls such Underwriter on account of any such loss, liability, claim, damage or expense arising out of any such defect or alleged defect in any preliminary prospectus if a copy of the Prospectus (exclusive of the documents incorporated by reference therein) shall not have been given or sent by such Underwriter with or prior to the written confirmation of the sale involved to the extent that (i) the Prospectus would have cured such defect or alleged defect and (ii) sufficient quantities of the Prospectus were timely made available to such Underwriter.

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(in addition to local counsel) to represent them and those other Underwriters and their respective officers, employees and controlling persons who may be subject to liability arising out of any claim in respect of which indemnity may be sought by the Underwriters against the Company under this Section 6 if, in the reasonable judgment of the Representatives, either (i) there is an actual or potential conflict between the position of the Company on the one hand and the Underwriters on the other hand or (ii) there may be defenses available to it or them that are different from or additional to those available to the Company (in any of which events the Company shall not have the right to direct the defense of such action on behalf of the Representatives with respect to such different defenses), in any of which events such reasonable fees and expenses shall be borne by the Company. No indemnifying party shall, without the prior written consent of the indemnified parties, settle or compromise or consent to the entry of any judgment with respect to any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever in respect of which indemnification or contribution could be sought under this Section 6 or Section 7 hereof (whether or not the indemnified parties are actual or potential parties thereto), unless such settlement, compromise or consent (i) includes an unconditional release of each indemnified party from all liability arising out of such litigation, investigation, proceeding or claim and (ii) does not include a statement as to or an admission of fault, culpability or a failure to act by or on behalf of any indemnified party.

     SECTION 7. Contribution. If the indemnification provided for in Section 6 hereof is for any reason unavailable to or insufficient to hold harmless an indemnified party in respect of any losses, liabilities, claims, damages or expenses referred to therein; then each indemnifying party shall contribute to the aggregate amount of such losses, liabilities, claims, damages and expenses incurred by such indemnified party, as incurred, (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other hand from the offering of the Securities pursuant to this Agreement or (ii) if the allocation provided by clause (i) is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (i) above but also the relative fault of the Company on the one hand and of the Underwriters on the other hand in connection with the statements or omissions, which resulted in such losses, liabilities, claims, damages or expenses, as well as any other relevant equitable considerations.

     The relative benefits received by the Company on the one hand and the Underwriters on the other hand in connection with the offering of the Securities pursuant to this Agreement shall be deemed to be in the same respective proportions as the total net proceeds from the offering of

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the Securities pursuant to this Agreement (before deducting expenses) received by the Company and the total underwriting discount received by the Underwriters, in each case as set forth on the cover of the Prospectus, bear to the aggregate initial public offering price of the Securities as set forth on such cover.

     The relative fault of the Company on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether any such untrue or alleged untrue statement of a material fact or omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

     The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 7 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to above in this Section 7. The aggregate amount of losses, liabilities, claims, damages and expenses incurred by an indemnified party and referred to above in this Section 7 shall be deemed to include any legal or other expenses reasonably incurred by such indemnified party in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue or alleged untrue statement or omission or alleged omission.

     Notwithstanding the provisions of this Section 7, no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of any such untrue or alleged untrue statement or omission or alleged omission.

     No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the ▇▇▇▇ ▇▇▇) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

     For purposes of this Section 7, each person, if any, who controls an Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as such Underwriter, and each director of the Company, each officer of the Company who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as the Company. The Underwriters’ respective obligations to contribute pursuant to this Section 7 are several in proportion to the principal amount of Securities set forth opposite their respective names in Schedule A hereto and not joint.

     SECTION 8. Representations, Warranties and Agreements to Survive Delivery. All representations, warranties and agreements contained in this Agreement or in certificates of officers of the Company or any of its subsidiaries submitted pursuant hereto, shall remain operative and in full force and effect, regardless of any investigation made by or on behalf of any

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Underwriter or controlling person, or by or on behalf of the Company, and shall survive delivery of and payment for the Securities to the Underwriters.

     SECTION 9. Termination of Agreement.

     SECTION 10. Default by One or More of the Underwriters. If one or more of the Underwriters shall fail at the Closing Time to purchase the Securities which it or they are obligated to purchase under this Agreement (the “Defaulted Securities”), the Representatives shall have the right, within 24 hours thereafter, to make arrangements for one or more of the non-defaulting Underwriters, or any other underwriters, to purchase all, but not less than all, of the Defaulted Securities in such amounts as may be agreed upon and upon the terms herein set forth; if, however, the Representatives shall not have completed such arrangements within such 24-hour period, then:

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amount thereof in the proportions that their respective underwriting obligations hereunder bear to the underwriting obligations of all non-defaulting Underwriters, or

     No action taken pursuant to this Section shall relieve any defaulting Underwriter from liability in respect of its default.

     In the event of any such default which does not result in a termination of this Agreement, either the Representatives or the Company shall have the right to postpone the Closing Time for a period not exceeding seven days in order to effect any required changes in the Registration Statement or Prospectus or in any other documents or arrangements. As used herein, the term “Underwriter” includes any person substituted for an Underwriter under this Section 10.

     SECTION 11 Notices. All notices and other communications hereunder shall be in writing and shall be deemed to have been duly given if mailed or transmitted by any standard form of telecommunication. Notices to the Underwriters shall be directed to the Representatives at:

and notices to the Company shall be directed to it at ▇▇▇▇ ▇▇▇▇▇ ▇▇▇▇▇▇▇▇ ▇▇▇▇, ▇▇▇▇▇▇▇, ▇▇▇▇▇▇▇▇▇, ▇▇▇▇▇, attention of ▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇, Vice President and Treasurer, or his successor.

     SECTION 12. Parties. This Agreement shall each inure to the benefit of and be binding upon the Underwriters and the Company and their respective successors. Nothing expressed or mentioned in this Agreement is intended or shall be construed to give any person, firm or corporation, other than the Underwriters and the Company and their respective successors and the controlling persons and officers and directors referred to in Sections 6 and 7 and their heirs and legal representatives, any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision herein contained. This Agreement and all conditions and provisions hereof are intended to be for the sole and exclusive benefit of the Underwriters and the Company and their respective successors, and said controlling persons and officers and directors and their heirs and legal representatives, and for the benefit of no other person, firm or corporation. No purchaser of Securities from any Underwriter shall be deemed to be a successor by reason merely of such purchase.

     SECTION 13. No Fiduciary Duty. The Company hereby acknowledges that the Underwriters will be acting pursuant to a contractual relationship on an arm’s length basis and in no event do the parties intend that the Underwriters act or be responsible as a fiduciary to the

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Company, its management, stockholders, creditors or any other person. The Company and the Underwriters each hereby expressly disclaim any fiduciary relationship and agree they are each responsible for making their own independent judgments with respect to any transactions entered into between them.

     SECTION 14. GOVERNING LAW AND TIME. THIS AGREEMENT AND ALL DISPUTES, CONTROVERSIES OR CLAIMS ARISING OUT OF OR RELATING TO THIS AGREEMENT OR A BREACH HEREOF SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK. EXCEPT AS OTHERWISE SET FORTH HEREIN, SPECIFIED TIMES OF DAY REFER TO NEW YORK CITY TIME.

     SECTION 15. Effect of Headings. The Article and Section headings herein and the Table of Contents are for convenience only and shall not affect the construction hereof.

     SECTION 16. Counterparts. This Agreement may be executed in one or more counterparts and, if executed in more than one counterpart, the executed counterparts shall constitute a single instrument.

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     If the foregoing is in accordance with your understanding of our agreement, please sign and return to the Company a counterpart hereof, whereupon this instrument, along with all counterparts, will become a binding agreement between the Underwriters and the Company in accordance with its terms.

For themselves and as Representatives of the other Underwriters named in Schedule A hereto.

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SCHEDULE A

SCHEDULE B

INTERSTATE POWER AND LIGHT COMPANY

5.50% SENIOR DEBENTURES DUE 2025

     1. The initial public offering price of the Securities shall be 99.414% of the principal amount thereof, plus accrued interest, if any, from the date of issuance.

     2. The purchase price to be paid by the Underwriters for the Securities shall be 98.539% of the principal amount thereof.

     3. The interest rate on the Securities shall be 5.50% per annum.

     4. The Company may redeem the Securities at any time at its option, in whole or in part, at a redemption price equal to the sum of the principal amount of the Securities to be redeemed, accrued interest on that principal amount to the redemption date and the make-whole amount, if any, with respect to the Securities to be redeemed. This sum is referred to as the redemption price.

     “Make-whole amount” means, in connection with the optional redemption, the excess, if any, of:

     “Reinvestment rate” means 0.20% plus the arithmetic mean of the yields under the headings “Week Ending” published in the most recent statistical release under the caption “Treasury Constant Maturities” for the maturity, rounded to the nearest month, corresponding to the remaining life to maturity, as of the payment date of the principal being redeemed. If no maturity exactly corresponds to such maturity, yields for the two published maturities most closely corresponding to such maturity will be calculated pursuant to the immediately preceding sentence and the reinvestment rate will be interpolated or extrapolated from such yields on a straight-line basis, rounding in each of the relevant periods to the nearest month. For purposes of calculating the reinvestment rate, the most recent statistical release published prior to the date of determination of the make-whole amount will be used.

     “Statistical release” means the statistical release designated “H.15(519)” or any successor publication which is published weekly by the Federal Reserve System and which establishes yields on actively traded United States government securities adjusted to constant maturities or, if such statistical release is not published at the time of any determination, then such other reasonably comparable index which shall be designated by the Company.

EXHIBIT A

FORM OF OPINION OF ▇▇▇▇▇ & ▇▇▇▇▇▇▇

     The Company is validly existing as a corporation under the laws of the State of Iowa.

     The Company has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus and to enter into and perform its obligations under the Purchase Agreement.

     The Company is duly qualified as a foreign corporation to transact business and is in good standing in each domestic jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

     The shares of issued and outstanding capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable; and, to the best of our knowledge, none of the outstanding shares of capital stock of the Company were issued in violation of preemptive or other similar rights of any securityholder of the Company.

     The Company has no “significant subsidiary” as defined in Rule 1-02 of Regulation S-X.

     The Company has all requisite corporate power and authority to execute and deliver the Purchase Agreement and to perform its obligations thereunder. The Purchase Agreement has been duly authorized, executed and delivered by the Company.

     The Indenture has been duly authorized, executed and delivered by the Company and (assuming the due authorization, execution and delivery thereof by the Trustee) constitutes a valid and binding agreement of the Company, enforceable against the Company in accordance with its terms, except as the enforcement thereof may be limited by bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting enforcement of creditors’ rights generally and except as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law).

     The Securities are in the form contemplated by the Indenture, have been duly authorized by the Company and, when executed by the Company and authenticated by the Trustee in the manner provided in the Indenture (assuming due authorization, execution and delivery of the Indenture by the Trustee), and issued and delivered against payment of the purchase price therefor will constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms, except as the enforcement thereof may be limited by bankruptcy, insolvency (including, without limitation, all laws relating to fraudulent transfers), reorganization, moratorium or similar laws affecting enforcement of creditors’ rights generally

and except as enforcement thereof is subject to general principles of equity (regardless of whether enforcement is considered in a proceeding in equity or at law), and will be entitled to the benefits of the Indenture.

     The Indenture has been duly qualified under the 1939 Act.

     The Registration Statement, including any Rule 462(b) Registration Statement, has been declared effective under the 1933 Act; any required filing of the Prospectus pursuant to Rule 424(b) has been made in the manner and within the time period required by Rule 424(b); and, to the best of our knowledge, no stop order suspending the effectiveness of the Registration Statement or any Rule 462(b) Registration Statement has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or threatened by the Commission.

     The Registration Statement, including any Rule 462(b) Registration Statement, the Prospectus, excluding the documents incorporated by reference therein, and each amendment or supplement to the Registration Statement and Prospectus, as of their respective effective or issue dates (other than the financial statements, statistical data and supporting schedules included therein or omitted therefrom, as to which we need express no opinion) complied as to form in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations.

     The documents incorporated by reference in the Prospectus (other than the financial statements and supporting schedules included therein or omitted therefrom, as to which we need express no opinion), when they were filed with the Commission complied as to form in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission thereunder.

     The Securities and the Indenture conform as to legal matters in all material respects to the descriptions thereof contained in the Prospectus.

     To the best of our knowledge, other than as set forth in the Prospectus, there is not pending or threatened any action, suit, proceeding, inquiry or investigation, to which the Company or any subsidiary thereof is a party, or to which the property of the Company or any subsidiary thereof is subject, before or brought by any court or governmental agency or body, which might reasonably be expected to result in a Material Adverse Effect, or which might reasonably be expected to materially and adversely affect the properties or assets thereof or the consummation of the transactions contemplated in the Purchase Agreement or the performance by the Company of its obligations thereunder.

     The information in the Prospectus under “Description of the Senior Debentures” and in the Registration Statement under Item 15, to the extent that it constitutes matters of law, summaries of legal matters, the Company’s charter or legal proceedings, or legal conclusions, has been reviewed by us and is correct in all material respects.

     All descriptions in the Registration Statement of franchises, contracts, indentures, mortgages, loan agreements, notes, leases or other instruments to which the Company or any of its subsidiaries are a party, to the extent that they describe legal matters, are accurate in all

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material respects; and to the best of our knowledge, there are no such documents required to be described or referred to in the Registration Statement or to be filed as exhibits thereto other than those described or referred to therein or filed or incorporated by reference as exhibits thereto.

     To the best of our knowledge, the Company is not in violation of its charter or by-laws and no default by the Company exists in the due performance or observance of any material obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, loan agreement, note, lease or other agreement or instrument that is described or referred to in the Registration Statement or Prospectus or filed or incorporated by reference as an exhibit to the Registration Statement.

     No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any domestic court or governmental authority or agency (other than such as have already been obtained and such as may be required under the applicable securities laws of the various jurisdictions in which the Securities will be offered or sold and the 1935 Act (solely with respect to filings required to be made with the Commission subsequent to the Closing Time), as to which we need express no opinion) is necessary or required in connection with the due authorization, execution and delivery of the Purchase Agreement or the Indenture by the Company or for the offering, issuance, sale or delivery of the Securities.

     The execution, delivery and performance of the Purchase Agreement, the DTC Agreement, the Indenture and the Securities and the consummation of the transactions contemplated in the Purchase Agreement and in the Registration Statement (including the issuance and sale of the Securities and the use of the proceeds from the sale of the Securities as described in the Prospectus under the caption “Use Of Proceeds”) and compliance by the Company with its obligations under the Purchase Agreement, the Indenture and the Securities do not and will not, whether with or without the giving of notice or lapse of time or both, conflict with or constitute a breach of, or default or Repayment Event (as defined in Section 1(a)(xiii) of the Purchase Agreement) under or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company pursuant to those agreements set forth on Schedule A hereto (except for such conflicts, breaches or defaults or liens, charges or encumbrances that would not have a Material Adverse Effect), nor will such action result in any violation of the provisions of the charter or by-laws, or similar organizational documents, of the Company, or any applicable law, statute, rule, regulation, judgment, order, writ or decree, known to us, of any U.S. government, government instrumentality or court, having jurisdiction over the Company or any of its properties, assets or operations.

     The Company is not an “investment company” or an entity “controlled” by an “investment company,” as such terms are defined in the ▇▇▇▇ ▇▇▇.

     The issuance of the Securities complies with all requirements of the Securities and Exchange Commission’s Release No. 35-27930, 70-10249, dated as of December 28, 2004.

     Nothing has come to our attention that would lead us to believe that the Registration Statement or any amendment or supplement thereto (except for financial statements and schedules and other financial or statistical data included or incorporated by reference therein or omitted therefrom, as to which we need make no statement), at the time such Registration

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Statement or any such amendment became effective, contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading or that the Prospectus or any amendment or supplement thereto (except for financial statements and schedules and other financial or statistical data included or incorporated by reference therein or omitted therefrom, as to which we need make no statement), as of the date of the Prospectus, as of the date of any such amended or supplemented prospectus or at the Closing Time, included or includes an untrue statement of a material fact or omitted or omits to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

In rendering this opinion, we relied as to matters of fact (but not as to legal conclusions), to the extent we deemed proper, on certificates of responsible officers of the Company and public officials. We have also relied on the legal opinion of Iowa counsel for certain matters of Iowa corporate law and the Company’s in-house counsel for certain matters of state regulatory law.

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SCHEDULE A TO THE FORM OF OPINION OF ▇▇▇▇▇ & ▇▇▇▇▇▇▇

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EXHIBIT B

FORM OF OPINION OF IN-HOUSE COUNSEL TO THE COMPANY

     No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any state or local court or governmental authority or agency (other than such as may be required under the applicable securities laws of the various jurisdictions in which the Securities will be offered or sold, as to which I need express no opinion, and such as have already been obtained) is necessary or required under the laws of the States of Illinois, Iowa or Minnesota in connection with the due authorization, execution and delivery of the Purchase Agreement or the Indenture by the Company or for the offering, issuance, sale or delivery of the Securities.

     In rendering this opinion, I relied as to matters of fact (but not as to legal conclusions), to the extent I deemed proper, on certificates of responsible officers of the Company and public officials.