1
                                   FORM 8-A/A

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                FOR REGISTRATION OF CERTAIN CLASSES OF SECURITIES

                     PURSUANT TO SECTION 12(b) OR (g) OF THE

                         SECURITIES EXCHANGE ACT OF 1934


                                 SOUTHDOWN, INC.
             (Exact name of registrant as specified in its charter)

          LOUISIANA                                    72-0296500
(State or other jurisdiction               (I.R.S. Employer Identification No.)
     of incorporation)

                        1200 SMITH STREET, SUITE 2400
                          HOUSTON, TEXAS 77002-4486
             (Address of principal executive offices) (Zip Code)

      Securities to be registered pursuant to Section 12(b) of the Act:

        TITLE OF EACH CLASS                      NAME OF EACH EXCHANGE ON WHICH
         TO BE REGISTERED                        EACH CLASS IS TO BE REGISTERED
- ---------------------------------------          -----------------------------
Common Stock, par value $1.25 per share          New York Stock Exchange, Inc.
Preferred Stock Purchase Rights                  New York Stock Exchange, Inc.

                                   ----------

         If this form relates to the registration of a class of securities
pursuant to Section 12(b) of the Exchange Act and is effective pursuant to
general Instruction A.(c), check the following box. [X]

         If this form relates to the registration of a class of securities
pursuant to Section 12(g) of the Exchange Act and is effective pursuant to
General Instruction A.(d), check the following box. [ ]

         Securities Act registration statement file number to which this form
relates:

         .................... (if applicable)

         Securities to be registered pursuant to Section 12(g) of the Act:

                                  None



   2




ITEM 1. DESCRIPTION OF REGISTRANT'S SECURITIES TO BE REGISTERED.

         Item 1 is hereby amended by adding at the end of the description under
"Rights" the following:

                  In connection with the Agreement and Plan of Merger dated as
         of September 28, 2000 (the "Merger Agreement") by and among CEMEX, S.A.
         de C.V., a corporation organized under the laws of the United Mexican
         States ("Parent"), CENA Acquisition Corp., a Delaware corporation and
         an indirect subsidiary of Parent ("Purchaser"), and Southdown, Inc., a
         Louisiana corporation (the "Company"), as of September 28, 2000, the
         Company amended its Rights Agreement dated as of March 4, 1991 between
         the Company and American Stock Transfer and Trust Company, as successor
         Rights Agent (the "Rights Agreement"), by a First Amendment to Rights
         Agreement dated September 28, 2000 (the "Amendment"). Capitalized terms
         used herein and not otherwise defined have the meanings assigned to
         them in the Rights Agreement or the Amendment, as applicable.

                  The Amendment provides that, notwithstanding anything to the
         contrary contained in the Rights Agreement, neither Parent, Purchaser
         nor any of their Affiliates or Associates shall be deemed to be the
         Beneficial Owner of, or to beneficially own, any of the Common Stock by
         virtue of the approval, execution and delivery of the Merger Agreement,
         the option to purchase shares of Common Stock set forth in the Merger
         Agreement, the making of the Offer, or the consummation of the
         transactions contemplated by the Merger Agreement.

                  The Amendment also provides that, notwithstanding anything to
         the contrary contained in the Rights Agreement, the Rights may not be
         exercised at or after the acceptance for payment of shares of Common
         Stock in the Offer. The amendment also provides that the Rights, if
         they have not expired earlier, will expire upon the acceptance for
         payment of shares of Common Stock in the Offer.

                  Finally, the amendment provides that, notwithstanding anything
         to the contrary contained in the Rights Agreement, the approval,
         execution and delivery of the Merger Agreement (including any
         amendments thereto), the option to purchase shares of Common Stock set
         forth in the Merger Agreement, the making of the Offer or the
         consummation of the transactions contemplated by the Merger Agreement
         shall not cause (i) Purchaser, Parent or any of their Affiliates or
         Associates to be an Acquiring Person; (ii) a Stock Acquisition Date to
         occur; (iii) a Distribution Date to occur; or (iv) the Rights to become
         exercisable.

                  The Amendment is filed herewith as Exhibit 4.3 and is
         incorporated herein by reference. The foregoing description of the
         Amendment does not purport to be complete and is qualified in its
         entirety by reference to that Exhibit.


                                       -2-

   3



         Item 1 is hereby also amended by adding at the end of the description
under "Statutory Provision" the following:

                  In connection with the Merger Agreement, on September 28,
         2000, the Company amended its Bylaws to provide that the control shares
         provisions of the Louisiana Business Corporation Law do not apply to
         control share acquisitions of shares of the Company.


ITEM 2. EXHIBITS.

         Item 2 is hereby amended by adding the following thereto:


  Exhibit Number      Description of Exhibit

         3.3          Bylaws of the Company as amended through September
                      28, 2000, incorporated by reference to Exhibit 3.1 of
                      the Current Report on Form 8-K dated September 28,
                      2000.

         4.3          First Amendment to Rights Agreement dated September
                      28, 2000 between the Company and American Stock
                      Transfer and Trust Company, as Successor Rights
                      Agent, incorporated by reference to Exhibit (e)(29)
                      to the Schedule 14D-9 filed by Southdown, Inc. on
                      October 5, 2000.



                                       -3-

   4



                                    SIGNATURE

         Pursuant to the requirements of Section 12 of the Securities Exchange
Act of 1934, the registrant has duly caused this amendment to the registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized.


                                       SOUTHDOWN, INC.

                                       By: /s/ Patrick S. Bullard
                                           ------------------------------------
                                       Name:  Patrick S. Bullard
                                       Title: Senior Vice President -
                                              General Counsel and Secretary


Dated: October 20, 2000




                                       -4-

   5
                                INDEX OF EXHIBITS



Exhibit
Number         Description of Exhibit
- -------        ----------------------
            
  3.3          Bylaws of the Company as amended through September 28, 2000,
               incorporated by reference to Exhibit 3.1 of the Current Report on
               Form 8-K dated September 28, 2000.

  4.3          First Amendment to Rights Agreement dated September 28, 2000
               between the Company and American Stock Transfer and Trust
               Company, as Successor Rights Agent, incorporated by reference to
               Exhibit (e)(29) to the Schedule 14D-9 filed by Southdown, Inc. on
               October 5, 2000.