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:  September 2, 2005

                                          NOVARTIS AG


                                          By: /s/ Xxxxxx Xxxxxx
                                              ----------------------------------
                                              Name: Xxxxxx Xxxxxx
                                              Title: Authorized Signatory

                                          By: /s/ Xxxxx Xxxxxxxxx
                                              ----------------------------------
                                              Name: Xxxxx Xxxxxxxxx
                                              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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