THIRD  SUPPLEMENTAL  INDENTURE,  effective  as of December 31,
1999, to the  Indenture,  dated as of September 20, 1996, as heretofore  amended
and  supplemented  (the  "Indenture"),  between  Commonwealth  Industries,  Inc.
(formerly  Commonwealth  Aluminum  Corporation),  a  Delaware  corporation  (the
"Company"),  each of the Subsidiary  Guarantors (as defined  therein) and Harris
Trust and Savings Bank, as Trustee (the "Trustee").

                                    RECITALS:

                  The Indenture has heretofore been amended and  supplemented by
a First  Supplemental  Indenture,  dated as of November 12,  1996,  and a Second
Supplemental Indenture,  dated as of October 16, 1998. Subsequent to the date of
the Second Supplemental  Indenture and prior to the date hereof, the Company has
duly  organized  Alflex E1 LLC,  a  Delaware  limited  liability  company,  as a
Restricted  Subsidiary,  and it is proposed that this limited  liability company
(the "new subsidiary") become an additional Subsidiary  Guarantor,  as permitted
by Section 901(7) of the Indenture.

                  The Company,  each of the  Subsidiary  Guarantors  and the new
subsidiary  have  been  authorized  by  Board  Resolutions  to enter  into  this
supplemental indenture.

                  NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE
WITNESSETH:

                  For  and in  consideration  of the  premises,  it is  mutually
agreed,  for  the  equal  and  proportionate  benefit  of  all  Holders  of  the
Securities, as follows:

                                   ARTICLE ONE

                                   Definitions

                  For all purposes of this  supplemental  indenture,  unless the
context otherwise  requires,  the terms used herein shall have the same meanings
as in the Indenture.

                                   ARTICLE TWO

                              Subsidiary Guarantors

                  The new  subsidiary  is  hereby  subjected  to the  provisions
(including the  representations and warranties) of the Indenture as a Subsidiary
Guarantor, all as contemplated by Section 1303 of the Indenture.


<PAGE>


                  IN WITNESS  WHEREOF,  the  parties  hereto  have  caused  this
supplemental indenture to be duly executed, and their respective corporate seals
to be  hereunto  affixed  and  attested,  all as of the day and year first above
written.

                                    COMMONWEALTH INDUSTRIES, INC.
                                    COMMONWEALTH ALUMINUM CORPORATION
                                    COMMONWEALTH ALUMINUM CONCAST, INC.
                                    COMMONWEALTH ALUMINUM LEWISPORT, INC.
                                    COMMONWEALTH ALUMINUM SALES CORPORATION
                                    ALFLEX CORPORATION


                                    By:   ______________________________
                                          Mark V. Kaminski
                                          President and Chief Executive Officer

                                    ALFLEX E1 LLC

                                    By:  Alflex Corporation,
                                    as Managing Member



                                    By:   ______________________________
                                          Mark V. Kaminski
                                          President and Chief Executive Officer

Attest:



By:    ______________________________
       Secretary



<PAGE>



                                    HARRIS TRUST AND SAVINGS BANK,
                                    as Trustee



                                    By:   ______________________________
                                    Name:
                                    Title:

Attest:



By:    ______________________________
       Secretary


COMMONWEALTH OF KENTUCKY            )
                                                 )     ss.:
COUNTY OF JEFFERSON                              )

                  On the  _____  day of  ____________________,  1999,  before me
personally came Mark V. Kaminski,  to me known, who, being by me duly sworn, did
depose  and say that he is  President  and Chief  Executive  Officer  of each of
Commonwealth Industries,  Inc., Commonwealth Aluminum Corporation,  Commonwealth
Aluminum Concast,  Inc.,  Commonwealth  Aluminum Lewisport,  Inc.,  Commonwealth
Aluminum Sales Corporation and Alflex Corporation, corporations described in and
which  executed  the  foregoing  instrument;  that  he  knows  the  seal of said
corporations;  that the seals  affixed  to said  instrument  are such  corporate
seals; that they were so affixed by authority of the Boards of Directors of said
corporations, and that he signed his name thereto by like authority.


                                    ------------------------------



<PAGE>


STATE OF ILLINOIS                   )
                                    )     ss.:
COUNTY OF COOK                      )

                  On the  _____  day of  ____________________,  1999,  before me
personally came  ______________________________,  to me known,  who, being by me
duly sworn, did depose and say that she is a  ______________________________  of
Harris Trust and Savings Bank,  one of the  corporations  described in and which
executed the foregoing instrument;  that she knows the seal of said corporation;
that the seal affixed to said  instrument is such corporate seal; that it was so
affixed by authority of the Board of Directors of said corporation, and that she
signed her name thereto by like authority.


                                    ------------------------------