In France, national management programs focus research on understanding reproductive factors in Crassostrea gigas to confront problems of the oyster industry. However, little information has been documented in which reproductive patterns include sexual changes. The reproductive cycle of oysters at three sites of the Atlantic coast of France was examined from 1996 to 1998, and the seasonal variations in oocyte size-frequencies, and sex ratio were described. The results showed a synchronism within the population concerning reproductive behavior. Young oocytes are generated after spawning and show no apparent changes during winter. Growth of oocytes begins in spring and cells reach maturity in April-May and are ready for a single spawning season in June-July. Oocytes that were not released during spawning are reabsorbed within the gonad. The significant difference between sites is that spawning occurred 1 month later in the southern area. A modal analysis showed that oocyte populations in the sample individuals are primordially bimodal, but with polymodal occurrences in June-July, in some cases. Irregular alternative sexuality was detected at all sites, and hermaphrodites appear to be a transition phase that allows changes from male to female during early spring. Previous observations, together with the study of the development of oocyte cohorts over time, permit a hypothetical model concerning the kinetics of gametogenesis in C. gigas. The model suggests that primary oocytes are generated from energy supplied from degenerating, as well as young oocytes that do not reach the mature stage within the gonad during autumn-winter. It seems that, during vitellogenesis, there is disintegration of smaller cells coupled with transfer of energy to the larger oocytes, which continue to grow and mature.