SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                 FORM 10-K / A-1


|X|      ANNUAL  REPORT  PURSUANT  TO  SECTION  13 OR  15(d)  OF THE  SECURITIES
         EXCHANGE ACT OF 1934 - For the fiscal year ended December 31, 1999

                                       OR

|_|      TRANSITION  REPORT  PURSUANT  TO SECTION 13 OR 15(d) OF THE  SECURITIES
         EXCHANGE ACT OF 1934

                          Commission file number 1-640

                               NL INDUSTRIES, INC.
--------------------------------------------------------------------------------
             (Exact name of registrant as specified in its charter)

          New Jersey                                              13-5267260
-------------------------------                              -------------------
(State or other jurisdiction of                                 (IRS Employer
 incorporation or organization)                              Identification No.)

16825 Northchase Drive, Suite 1200, Houston, Texas               77060-2544
--------------------------------------------------           -------------------
   (Address of principal executive offices)                      (Zip Code)

Registrant's telephone number, including area code:             (281) 423-3300
                                                             -------------------

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

                                                        Name of each exchange on
      Title of each class                                    which registered
------------------------------                          ------------------------
Common stock ($.125 par value)                           New York Stock Exchange
                                                         Pacific Exchange

Securities registered pursuant to Section 12(g) of the Act:  None.

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during
the  preceding 12 months,  and (2) has been subject to such filing  requirements
for the past 90 days.     Yes X            No

Indicate by check mark if disclosure of delinquent  filers  pursuant to Item 405
of Regulation  S-K is not contained  herein,  and will not be contained,  to the
best of registrant's  knowledge,  in definitive proxy or information  statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. |X|

The  aggregate  market value of the voting stock held by  non-affiliates  of the
registrant at March 16, 2000 approximated $167 million.

There were 50,629,740 shares of common stock outstanding at March 16, 2000.

                      Documents incorporated by reference:

The  information  required by Part III is  incorporated  by  reference  from the
Registrant's  definitive  proxy  statement to be filed with the  Securities  and
Exchange Commission pursuant to Regulation 14A not later than 120 days after the
end of the fiscal year covered by this report.


<PAGE>



The  undersigned   Registrant  hereby  amends  the  following  items,  financial
statements, exhibits or other portions of its Annual Report on Form 10-K for the
year  ended  December  31,  1999 as set forth  below  and in the pages  attached
hereto:

Item 14.        EXHIBITS, FINANCIAL STATEMENT SCHEDULES,
                AND REPORTS ON FORM 8-K.

                Exhibit  No.  99.1,   Annual  Report  of  NL  Industries,   Inc.
                Retirement Savings Plan on Form 11-K for the year ended December
                31,  1999  (filed as an  amendment  to the  Registrant's  Annual
                Report on Form 10-K for the year ended December 31, 1999).




<PAGE>



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



                                                        NL INDUSTRIES, INC.
                                                   -----------------------------
                                                           (Registrant)



Dated:  June 27, 2000                       By:    /s/ Robert D. Hardy
                                                   -----------------------------
                                                   Robert D. Hardy
                                                   Vice President and Controller