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                                                                     EXHIBIT 5.1





                                  July 14, 2000



Board of Directors
Lexicon Genetics Incorporated
4000 Research Forest Drive
The Woodlands, Texas 77381

Ladies and Gentlemen:

         We have acted as counsel to Lexicon Genetics Incorporated (the
"Company") in connection with the Company's Registration Statement on Form S-8
(the "Registration Statement") relating to the registration under the Securities
Act of 1933, as amended, of the issuance of up to (i) 11,250,000 shares (the
"Equity Incentive Plan Shares") of the Company's common stock, par value $0.001
per share (the "Common Stock"), pursuant to the Company's 2000 Equity Incentive
Plan (the "Equity Incentive Plan"); and (ii) 600,000 shares (the "Director Plan
Shares" and, together with the Equity Incentive Plan Shares, the "Shares") of
Common Stock, pursuant to the Company's 2000 Non-Employee Directors' Stock
Option Plan (the "Director Plan" and, together with the Equity Incentive Plan,
the "Plans").

         In connection herewith, we have examined copies of such statutes,
regulations, corporate records and documents, certificates of public and
corporate officials and other agreements, contracts, documents and instruments
as we have deemed necessary as a basis for the opinion hereafter expressed. In
such examination, we have assumed the genuineness of all signatures, the
authenticity of all documents submitted to us as originals and the conformity
with the original documents of all documents submitted to us as copies. We have
also relied, to the extent we deem such reliance proper, on information supplied
by officers and employees of the Company with respect to various factual matters
material to our opinion.

         Based on the foregoing and having due regard for such legal
considerations as we deem relevant, we are of the opinion that the Shares have
been duly authorized, and that such Shares will, when issued in accordance with
the terms of the Plans, be duly authorized, fully paid and validly issued.

         This opinion is limited in all respects to the General Corporation Law
of the State of Delaware and the laws of the United States of America insofar as
such laws are applicable.

         We hereby consent to the use of this opinion as an exhibit to the
Registration Statement. In giving this consent, we do not thereby admit that we
are within the category of persons whose consent is required under Section 7 of
the Securities Act or the rules and regulations of the Securities and Exchange
Commission thereunder.


                                                     Very truly yours,


                                                     ANDREWS & KURTH L.L.P.