[GRAPHIC OMITTED] [FOLEY & LARDNER LOGO]

                                          FOLEY & LARDNER
                                          777 East Wisconsin Avenue, Suite 3800
                                          Milwaukee, Wisconsin  53202-5306
                                          414.271.2400 TEL
                       October 1, 2003    414.297.4900  FAX
                                          www.foleylardner.com


                                          CLIENT/MATTER NUMBER
                                          026162-0101

Interstate Power and Light Company
Alliant Energy Tower
200 First Street, SE
Cedar Rapids, Iowa  52401
Ladies and Gentlemen:

     We have acted as counsel for Interstate Power and Light Company, an Iowa
corporation (the "Company"), in connection with the preparation of a
Registration Statement on Form S-3 (the "Registration Statement"), including the
Prospectus constituting a part thereof (the "Prospectus"), to be filed with the
Securities and Exchange Commission under the Securities Act of 1933, as amended
(the "Securities Act"), and relating to the issuance and sale by the Company of
up to $100,000,000 aggregate principal amount of senior unsecured debt
securities (the "Debentures") in the manner set forth in the Registration
Statement. The Debentures will be issued under the Indenture, dated as of August
20, 2003 (the "Indenture"), between the Company and Bank One Trust Company,
National Association, as Trustee, and an officer's certificate (the "Officer's
Certificate") establishing the terms and providing for the issuance of the
Debentures.

     As counsel to the Company in connection with the proposed issue and sale of
the Debentures, we have examined: (a) the Registration Statement, including the
Prospectus, and the exhibits (including those incorporated by reference)
constituting a part of the Registration Statement; (b) the Company's Restated
Articles of Incorporation and Bylaws, each as amended to date; (c) the
Indenture; and (d) such other proceedings, documents and records as we have
deemed necessary to enable us to render this opinion.

     In our examination of the above-referenced documents, we have assumed the
genuineness of all signatures, the authenticity of all documents, certificates
and instruments submitted to us as originals and the conformity with the
originals of all documents submitted to us as copies.

     Based upon the foregoing, we are of the opinion that:

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

     2. The Debentures, when executed, authenticated and issued in accordance
with the resolutions adopted by the Board of Directors of the Company on August
20, 2003 and in the manner and for the consideration contemplated by the
Registration Statement and the Prospectus, will be legally issued and valid and
binding obligations of the Company enforceable in accordance with their terms,
except as enforcement thereof may be limited by bankruptcy or other applicable
laws affecting the enforcement of creditors' rights generally or the application
of equitable


BRUSSELS   DETROIT        MILWAUKEE     SAN DIEGO           TAMPA
CHICAGO    JACKSONVILLE   ORLANDO       SAN DIEGO/DEL MAR   WASHINGTON, D.C.
DENVER     LOS ANGELES    SACRAMENTO    SAN FRANCISCO       WEST PALM BEACH
           MADISON                      TALLAHASSEE


Interstate Power and Light Company
October 1, 2003
Page 2

principles; provided, that prior to the issuance of the Debentures there shall
be taken various proceedings in the manner contemplated by us as counsel, which
include the following:

          a. The completion of the requisite procedure under the applicable
     provisions of the Securities Act and the Trust Indenture Act of 1939, as
     amended;

          b. The compliance with all regulatory authorizations relating to the
     issuance and sale of the Debentures;

          c. The further authorization by one or more specified senior executive
     officers of the Company of the Officer's Certificate relating to the
     Debentures, issuance of the Debentures and related matters; and

          d. The execution and delivery of the Officer's Certificate and the
     taking of other actions provided in the Indenture with respect to the
     issuance of unsecured debt securities thereunder.

     We hereby consent to the reference to our firm under the caption "Legal
Matters" in the Prospectus which is filed as part of the Registration Statement,
and to the filing of this opinion as an exhibit to the Registration Statement.
In giving this consent, we do not admit that we are "experts" within the meaning
of Section 11 of the Securities Act or within the category of persons whose
consent is required by Section 7 of the Securities Act.

                                Very truly yours,

                               /s/ Foley & Lardner