EXHIBIT 5
                               RIORDAN & MCKINZIE
                         A PROFESSIONAL LAW CORPORATION

                         300 S. GRAND AVENUE, 29TH FLOOR
                       LOS ANGELES, CALIFORNIA 90071-3155



                                October 26, 2000
                                                                      21-088-001

Tetra Tech, Inc.
670 N. Rosemead Boulevard
Pasadena, California  91107-2190

Ladies and Gentlemen:

                  We have acted as counsel to Tetra Tech, Inc., a Delaware
corporation (the "Company"), in connection with the registration under the
Securities Act of 1933, as amended (the "1933 Act"), of 202,376 authorized and
previously issued shares of the Common Stock (the "Shares"), $.01 par value per
share, of the Company to be sold by certain selling stockholders. This opinion
is delivered to you in accordance with the requirements of Item 601(b)(5) of
Regulation S-K under the 1933 Act in connection with the Registration Statement
on Form S-3, including all pre-effective and post-effective amendments thereto
(the "Registration Statement"), for the aforementioned sale, filed with the
Securities and Exchange Commission (the "Commission") under the 1933 Act.

                  In rendering the opinion set forth herein, we have made such
investigations of fact and law, and examined such documents and instruments, or
copies thereof established to our satisfaction to be true and correct copies
thereof, as we have deemed necessary under the circumstances.

                  Based upon the foregoing and such other examination of law and
fact as we have deemed necessary, and in reliance thereon, we are of the opinion
that, the Shares are duly authorized, validly issued, fully paid and
non-assessable.

                  We hereby consent to the filing of this opinion as an exhibit
to the Registration Statement and to the reference to this firm under the
caption "Legal Matters" in the Prospectus which is a part of the Registration
Statement. In giving such consent, we do not thereby admit that we are in the
category of persons whose consent is required under Section 7 of the 1933 Act or
the rules and regulations of the Commission thereunder.

                                      Very truly yours,

                                      /s/ Riordan & McKinzie