Exhibit 5.1

                             ___________ ____, 1998

Clark USA, Inc.
8182 Maryland Avenue
St. Louis, Missouri 63105

          Re:  Clark USA, Inc.
               Registration Statement on Form S-4
               ----------------------------------

Dear Ladies and Gentlemen:

          We have represented Clark USA, Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing with the Securities
and Exchange Commission under the Securities Act of 1933, as amended, of a
Registration Statement on Form S-4 (the "Registration Statement") relating to
the Company's 11 1/2% New Senior Cumulative Exchangeable Preferred Stock (the
"New Exchangeable Preferred Stock"), to be issued under the Exchange and
Registration Rights Agreement (the "Registration Agreement"), providing for the
Exchange Offer.

          In connection with our representation, we have examined such corporate
and other records, instruments, certificates and documents as we consider
necessary to enable us to express the opinions set forth below.

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

          1.  The Company is a corporation duly organized and validly existing
in good standing under the laws of the State of Delaware.

          2.  The Registration Agreement constitutes a valid and binding
obligation of the Company enforceable against the Company in accordance with its
terms, subject to applicable bankruptcy, insolvency, reorganization, moratorium
and other laws affecting the enforceability of creditors' rights generally and
to court decisions with respect thereto and to general principles of equity
(regardless of whether such enforceability is considered in a proceeding in
equity or at law).

          3.  The New Exchangeable Preferred Stock, upon the due execution,
authentication, issuance and delivery thereof in accordance with the
Registration Agreement, will constitute valid and legally binding obligations of
the Company entitled to the benefits provided by the Registration Agreement, and
enforceable in accordance with its terms, subject to applicable bankruptcy,
insolvency, reorganization, moratorium and other laws affecting the
enforceability of creditors' rights generally and to court decisions with
respect thereto and to general principles of equity (regardless of whether such
enforceability is considered in a proceeding in equity or at law).

          We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to all references to this firm in such Registration
Statement.

                                            Very truly yours,



                                            Mayer, Brown & Platt