In the pacific oyster, successive irregular protandrous hermaphrodite, the mode and period of sex determination remain unknown. This thesis consisted in specifying the molecular pathway and the temporal window of this process in adult and during development as well as its epigenetic and environmental potential regulations. Two new molecular actors respectively male and female were identified, Cg-SoxE and Cg-β-catenin. The natural antisense transcript of Cg-Foxl2 was also characterized; it may take part in male gonadic differentiation by inhibiting the expression of Cg-Foxl2 through duplex formation. In adult, expressions of these four factors analyzed by qRT-PCR and by in situ hybridization suggest that sex determination would take place from stage III of a gametogenetic cycle to stage 0 of the following cycle. A model of molecular pathway in adult proposes a balance of expressions between male and female factors characterized up to now (including Cg-DMl, actor of male gonadic differentiation). This balance could be regulated by epigenetic mechanisms as is the case for Cg-Foxl2. In spat, expression profiles of these same factors (including Cg-DMl) show a balance of expressions between male and female actors modulated by temperature and in accordance with sex-ratios, suggesting a temperature-dependent sex determination. A model of molecular pathway proposed in spat raised at 18°C in optimal nutritional conditions suggests that sex determination would take place around 40-44 dpf, before first appearance of collecting ducts around 49 dpf.