EXHIBIT 5.1

                       [Letterhead of Thompson Coburn LLP]

August 27, 2004

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

Re:    Post-Effective Amendment No. 1 to Registration Statement on Form S-8 -
       1,000,000 additional shares of Insituform Technologies, Inc. Class A
       common stock, $0.01 par value per share and attached Preferred Stock
       Purchase Rights

Ladies and Gentlemen:

We have acted as counsel to you (the "Company") in connection with the
preparation and filing of Post-Effective Amendment No. 1 to the Registration
Statement on Form S-8 (File No. 333-66714) (together with Post-Effective
Amendment No. 1, the "Registration Statement") under the Securities Act of 1933,
as amended (the "Securities Act"), covering an additional 1,000,000 shares of
the Company's common stock, par value $.01 per share ("Shares"), together with
one attached Preferred Stock Purchase Right per share, which may be issued
pursuant to the Insituform Technologies, Inc. Amended and Restated 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, 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