FOURTH AMENDMENT TO
                                CREDIT AGREEMENT


         This FOURTH AMENDMENT TO CREDIT AGREEMENT (this "Amendment") is entered
into  as of  August  20,  1999,  by and  between  Smithway  Motor  Xpress,  Inc.
("Borrower"),  Smithway  Motor Xpress Corp. as Guarantor (the  "Guarantor")  and
LaSalle Bank National  Association,  formerly known as LaSalle National Bank, as
Lender (the "Lender").

                              W I T N E S S E T H:

         WHEREAS, the Borrower and the Guarantor entered into a Credit Agreement
dated as of September 3, 1997, a First Amendment to Credit Agreement dated as of
March 1, 1998, a Second Amendment to Credit Agreement dated as of March 15, 1998
and a  Third  Amendment  to  Credit  Agreement  dated  as of  October  30,  1998
(collectively referred to as the "Agreement"); and

         WHEREAS, the Borrower has requested certain modifications to the Agree-
ment and the Lender is willing to do so on the following terms and conditions;
and

         NOW, THEREFORE,  in consideration of the mutual agreements,  provisions
and covenants contained herein, the parties agree as follows:

         1.  Unless  otherwise  stated  herein,  all  of the  capitalized  terms
contained  in this  document  shall have the same  meanings as  contained in the
Agreement.

         2.  Section 6.9 of the Agreement is hereby deleted and in lieu thereof
is inserted the following:

         The Borrower shall not incur  Indebtedness  in excess of $75,000,000.

         3. The  definition of Revolving  Termination  Date appearing in Section
9.1 hereof is hereby deleted and in lieu thereof is inserted the following:

                           "Revolving  Termination  Date"  means the  earlier to
                           occur of: (a)  September  1, 2002 unless  extended in
                           writing by all the parties  hereto;  and (b) the date
                           on which the  Aggregate  Revolving  Commitment  shall
                           terminate in accordance  with the  provisions of this
                           Agreement.

         4. Borrower shall pay 50% of the legal fees incurred in connection with
the preparation of this Amendment and the documents and instruments  referred to
herein,  and shall pay 100% of all out of pocket costs incurred by the Lender or
its attorneys.


         5.  Borrower  expressly  acknowledges  and agrees that all  collateral,
security  interests,  liens,  pledges,  and  mortgages  heretofore,  under  this
Amendment, or hereafter granted to Lender, including,  without limitation,  such
collateral,  security interests,  liens, pledges and mortgages granted under the
Agreement,  and all other supplements to the Agreement,  extend to and cover all
of the  obligations  of Borrower to Lender,  now existing or  hereafter  arising
including,  without limitation,  those arising in connection with the Agreement,
as amended by this Amendment,  upon the terms set forth in such agreements,  all
of which security interests,  liens, pledges, and mortgages are hereby ratified,
reaffirmed, confirmed and approved.




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         6.  Borrower  represents  and  warrants  to Lender  that (i) it has all
necessary  power and authority to execute and deliver this Amendment and perform
its  obligations  hereunder,  (ii) this Amendment and the Agreement,  as amended
hereby,  constitute the legal, valid and binding obligations of Borrower and are
enforceable  against  Borrower in  accordance  with their  terms,  and (iii) all
representations  and  warranties  of Borrower  contained  in the  Agreement,  as
amended,  and all other  agreements,  instruments  and other  writings  relating
thereto, are true, correct and complete as of the date hereof.

         7.  The  parties  hereto  acknowledge  and  agree  that the  terms  and
provisions  of  this  Amendment  amend,  add to  and  constitute  a part  of the
Agreement.  Except  as  expressly  modified  and  amended  by the  terms of this
Amendment,  all of the other terms and conditions of the Agreement,  as amended,
and  all  documents   executed  in  connection   therewith  or  referred  to  or
incorporated  therein  remain in full force and effect and are hereby  ratified,
reaffirmed, confirmed and approved.

         8. If there is an express  conflict between the terms of this Amendment
and the terms of the  Agreement,  or any of the other  agreements  or  documents
executed in connection  therewith or referred to or  incorporated  therein,  the
terms of this Amendment shall govern and control.

         9. This Amendment may be executed in one or more counterparts,  each of
which shall be deemed to be an original.

         10. This Amendment was executed and delivered in Chicago,  Illinois and
shall be governed by and  construed in  accordance  with the  internal  laws (as
opposed to conflicts of law provisions) of the State of Illinois.




<PAGE>


         IN WITNESS WHEREOF, this Amendment has been duly executed as of the day
and year specified at the beginning hereof.

                                    SMITHWAY MOTOR XPRESS, INC., as Borrower


                                    By:/s/G. Larry Owens
                                    Title:Executive Vice President, Chief
                                    Financial Officer & Chief Operating Officer

                                    Address Notice:
                                    P.O. Box 404
                                    Fort Dodge, Iowa 50501
                                    Attn: G. Larry Owens
                                    Facsimile: (515) 576-3304
                                    Tel: (515) 576-7418

                                    LASALLE BANK NATIONAL ASSOCIATION,
                                    as Lender

                                    By:/s/David A. Chaika
                                    Title:Commercial Banking Officer

                                    Address notices and Lending Office::
                                    135 S. LaSalle
                                    Chicago, Illinois 60603
                                    Attn: Mr. Bruce Linger
                                    Facsimile: (312) 904-6150
                                    Tel: (312) 904-8356