JOINT FILING AGREEMENT

                                                                       EXHIBIT 1

                             JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as
amended, each of the persons named below agrees to the joint filing of a
Statement on Schedule 13D (including amendments thereto) with respect to the
common stock, par value $0.001 per share, of Idenix Pharmaceuticals, Inc., a
Delaware corporation.

Date:  August 6, 2004

                                   NOVARTIS AG

                                   By: /s/ Xxxxx Xxxxxxxxx
                                        -----------------------------------
                                       Name: Xxxxx Xxxxxxxxx
                                       Title: Authorized Signatory

                                   By: /s/ Xxxx Xxxxxxx
                                       ------------------------------------
                                       Name: Xxxx Xxxxxxx
                                       Title: Authorized Signatory

                                   NOVARTIS PHARMA AG

                                   By: /s/ Xxxxxx X. Xxxxxx
                                       ------------------------------------
                                       Name: Xxxxxx X. Xxxxxx
                                       Title: General Counsel

                                   By: /s/ Xxxxxx X. Xxxxx
                                       ------------------------------------
                                       Name: Xxxxxx X. Xxxxx
                                       Title: Head Operational Treasury

                                   NOVARTIS BIOVENTURES LTD.

                                   By: /s/ Xxxx Xxxx
                                       ------------------------------------
                                       Name: Xxxx Xxxx
                                       Title: Member of the Board of Directors

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