Exhibit 10.18

                  THIRD AMENDMENT TO SECURED CREDIT AGREEMENT



Harris Trust and Savings Bank
Chicago, Illinois

Mercantile Bank National Association
St. Louis, Missouri

Ladies and Gentlemen:

     Reference is hereby made to that certain Secured Credit  Agreement dated as
of September 18, 1998 (as heretofore  amended the "Credit  Agreement") among the
undersigned, Maverick Tube Corporation, a Delaware corporation (the "Borrower"),
you (the "Banks") and Harris Trust and Savings Bank, as agent for the Banks (the
"Agent").  All defined  terms used herein  shall have the same meaning as in the
Credit Agreement unless otherwise defined herein.

     The  Borrower,  the  Agent and the Banks  wish to amend  certain  financial
covenants  contained in the Credit  Agreement and to modify  certain other terms
and conditions of the Credit Agreement, all on terms and conditions set forth in
this Amendment.

SECTION 1.     AMENDMENT TO CREDIT AGREEMENT

     Upon satisfaction of all of the conditions precedent set forth in Section 2
hereof, the Credit Agreement shall be amended as follows:

     1.1       Section  7.10 of the Credit  Agreement  is hereby  amended in its
               entirety and as so amended shall be restated to read as follows:

               Section 7.10.  Consolidated Tangible Net Worth. The Borrower will
               maintain  Consolidated  Tangible  Net Worth in an amount not less
               than (1)  $70,000,000  at all times from the date hereof  through
               September  30,  1998  and (b) at all  times  during  each  fiscal
               quarter of the  Borrower  thereafter,  in an amount not less than
               the Minimum  Required Amount.  For the purposes hereof,  the term
               "minimum  Required  Amount" shall mean an amount equal to the sum
               of (i) the Minimum  Required  Amount required to be maintained by
               the Borrower  during the  immediately  preceding  fiscal quarter,
               plus (ii) 75% of the Borrower's  Consolidated Net Income (but not
               less than zero) for such fiscal  quarter  then ended,  plus (iii)
               for the fiscal  quarter of the  Borrower  ending on December  31,
               1999, an amount equal to $26,175,000.


     1.2       Section  7.26 of the Credit  Agreement  is hereby  amended in its
               entirety and as so amended shall be restated to read as follows:

               Section 7.26.  Capital  Expenditures.  The Borrower will not, and
               will not permit a Subsidiary  to, expend or become  obligated for
               capital  expenditures  (as defined and  classified  in accordance
               with  Generally  accepted  accounting   principles   consistently
               applied but in any events including the liability of the Borrower
               and its  Subsidiaries  in respect of  Capitalized  Leases) in any
               fiscal year in an amount in the  aggregate  for the  Borrower and
               all of its  Subsidiaries  in  excess  of (i) for  the  Borrower's
               fiscal  year  ended  September  30,  1999,  an  amount  equal  to
               $13,000,000,  (ii) for the Borrower's fiscal year ended September
               30,  2000,  an  amount  equal to  $50,000,000  and (iii) for each
               fiscal year of the Borrower ending thereafter, an amount equal to
               $8,000,000.


SECTION 2.     CONDITIONS PRECEDENT

     The  effectiveness  of this Amendment is subject to the satisfaction of all
of the followings conditions precedent:

     2.1       The  Borrower,  the Agent and the Banks shall have  executed this
               Amendment (such execution may be in several  counterparts and the
               several parties hereto may execute on separate counterparts).

     2.2       A  guarantor's  Consent  for the  benefit of the Banks shall have
               been  executed and  delivered by each  Guarantor to the Agent,  a
               form of which is attached hereto.

     2.3       The Borrower  shall be in full  compliance  with all of the terms
               and  conditions of the Loan  Documents and no Event of Default or
               Potential   Default   shall  have   occurred  and  be  continuing
               thereunder or shall result after giving effect to this Amendment.

     2.4       Legal  matters  incident to the  execution  and  delivery of this
               Amendment  shall be  satisfactory  to each of the Banks and their
               legal counsel.


SECTION 3.     REPRESENTATIONS AND WARRANTIES.

     The Borrower,  by its  execution of this  Amendment,  hereby  certifies and
warrants the following:

     (a)       each of the representations and warranties set forth in Section 5
               of the Credit Agreement is true and correct as of the date hereof
               as if made on the date  hereof,  except that the  representations
               and warranties made under Section 5.2 shall be deemed to refer to
               the most  recent  annual  report  furnished  to the  Banks by the
               Borrower; and

     (b)       the  Borrower  is in full  compliance  with all of the  terms and
               conditions  of the  Credit  Agreement  and no Event of Default or
               Potential Default has occurred and is continuing thereunder.


SECTION 4.     MISCELLANEOUS

     4.1       The Borrower has  heretofore  executed and delivered to the Agent
               the  Security  Agreement  and the  Borrower  hereby  agrees  that
               notwithstanding  the execution and delivery hereof, such Security
               Agreement  shall be and  remain in full force and effect and that
               any rights and remedies of the Agent  thereunder,  obligations of
               the  Borrower  thereunder  and any  liens or  security  interests
               created or provided  for  thereunder  shall be and remain in full
               force and effect,  shall not be affected,  impaired or discharged
               thereby and shall  remain in full force and effect,  shall not be
               affected,  impaired or discharged thereby and shall secure all of
               its  indebtedness,  obligations  and liabilities to the Agent and
               the Banks under the Credit  Agreement as amended hereby.  Nothing
               herein  contained  shall  in any  manner  affect  or  impair  the
               priority of the liens and security interests created and provided
               for by the Security  Agreement as to the indebtedness which would
               be secured thereby prior to giving effect hereto.

     4.2       Reference  to this  specific  Amendment  need  not be made in any
               note, document, letter,  certificate,  any security agreement, or
               any  communication  issued or made pursuant to or with respect to
               the Credit Agreement, any reference to the Credit Agreement being
               sufficient to refer to the Credit Agreement as amended hereby.

     4.3       This Amendment may be executed in any number of counterparts, and
               by the different the different parties on different counterparts,
               all of which taken  together  shall  constitute  one and the same
               agreement.  Any of the parties  hereby may execute this agreement
               by signing  any such  counterpart  and each of such  counterparts
               shall  for  all  purposes  be  deemed  to  be an  original.  This
               agreement  shall be governed by the internal laws of the State of
               Illinois.

     4.4       The Borrower  agrees to pay all  reasonable  costs and  expenses,
               including  without  limitations  attorneys fees,  incurred by the
               Agent and each of the Banks in connection  with the  preparation,
               negotiation,  execution  and delivery of this  Amendment  and the
               other documents contemplated hereby.

Upon acceptance hereof by the Agent and the Banks in the manner  hereinafter set
forth,  this  Amendment  shall  be  a  contract  between  us  for  the  purposes
hereinabove set forth.

Dated as of December  8, 1999

                                   MAVERICK TUBE CORPORATION
                                       By:  /s/ Barry R. Pearl
                                           -------------------------
                                       Its:  Chief Financial Officer

Accepted and agreed to as of the day and year last above written

                                   HARRIS TRUST AND SAVINGS BANK
                                     Individually and as Agent

                                       By:  /s/ Bonnie A. Polic
                                           -------------------------
                                       Its:  Vice President


                                   MERCANTILE BANK NATIONAL ASSOCIATION

                                       By:  /s/ David Higbee
                                           -------------------------
                                       Its:  Vice President