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                       [Letterhead of Thompson Coburn LLP]


August 2, 2001



Insituform Technologies, Inc.
702 Spirit 40 Park Drive
Chesterfield, Missouri  63005

Re:      Registration Statement on Form S-8 - 1,000,000 shares of Insituform
         Technologies, Inc.
         Class A common stock, $0.01 par value per share

Ladies and Gentlemen:

         With reference to the Registration Statement on Form S-8 (the
"Registration Statement") to be filed by Insituform Technologies, Inc., a
Delaware corporation (the "Company"), on August 3, 2001, with the Securities and
Exchange Commission pursuant to the Securities Act of 1933, as amended, relating
to the proposed issuance by the Company of up to 1,000,000 shares of the
Company's Class A common stock, $0.01 par value per share (the "Shares"),
pursuant to the Insituform Technologies, Inc. 2001 Employee Equity Incentive
Plan (the "Plan"), we have examined such corporate records of the Company, such
laws and such other information as we have deemed relevant, including the
Company's Restated Certificate of Incorporation, as amended, By-Laws, as
amended, resolutions adopted by the Board of Directors relating to such
issuance, certificates received from state officials and statements we have
received from officers and representatives of the Company. In delivering this
opinion, we have assumed the genuineness of all signatures, the authenticity of
all documents submitted to us as originals, the conformity to the originals of
all documents submitted to us as certified, photostatic or conformed copies, the
authenticity of originals of all such latter documents and the correctness of
statements submitted to us by officers and representatives of the Company.

         Based solely on the foregoing, we are of the opinion that:

         1.       The Company is duly incorporated and is validly existing under
                  the laws of the State of Delaware; and

         2.       The Shares to be issued by the Company pursuant to the Plan
                  have been duly authorized and, when issued by the Company in
                  accordance with the Plan, will be validly issued, fully paid
                  and nonassessable.

         We consent to the filing of this opinion as an exhibit to the
Registration Statement. We further consent to the filing of copies of this
opinion with agencies of such states and other jurisdictions as you deem
necessary in the course of complying with the laws of the states and
jurisdictions regarding the sale and issuance of the Shares in accordance with
the Registration Statement.

                                          Very truly yours,

                                          /s/ Thompson Coburn LLP