JOINT FILING AGREEMENT

                                                                       EXHIBIT A

                             JOINT FILING AGREEMENT

         We, the signatories of Amendment 6 to the statement on Schedule 13D
filed with respect to the Common Stock of Quanta Services, Inc., to which this
Agreement is attached, hereby agree that such statement is, and any amendments
thereto filed by any of us will be, filed on behalf of each of us pursuant to
and in accordance with the provisions of Rule 13d-1(k) of the Securities
Exchange Act of 1934.


         Dated: December 13, 2004.


                                       FIRST RESERVE FUND IX, L.P.

                                       By: First Reserve GP IX, L.P., its
                                           General Partner,

                                           By: First Reserve GP IX, Inc., its
                                               General Partner


                                           By: /s/ Xxxxxx X. Xxxxxxx
                                               ---------------------------------
                                               Name:  Xxxxxx X. Xxxxxxx
                                               Title: Managing Director


                                       FIRST RESERVE GP IX, L.P.

                                       By: First Reserve GP IX, Inc., its
                                           General Partner


                                           By: /s/ Xxxxxx X. Xxxxxxx
                                               ---------------------------------
                                               Name:  Xxxxxx X. Xxxxxxx
                                               Title: Managing Director


                                       FIRST RESERVE GP IX, INC.


                                       By: /s/ Xxxxxx X. Xxxxxxx
                                          --------------------------------------
                                          Name:  Xxxxxx X. Xxxxxxx
                                          Title: Managing Director