Mortality outbreaks in adult oysters (Crassostrea gigas) have been reported since 1994 in Normandy. Recent work has shown that triploid oysters tend to be subject to higher mortality than diploids. Many parameters could be involved in this phenomenon including pathogens, environmental conditions, the physiological and genetic states of the oysters. With respect to reproductive physiology, studies showed that triploid oysters have gamete production. In order to understand the abnormal mortalities, we focused on gametogenesis and more specifically on hemocyte infiltration in gonadal tubules. Fourteen batches of oysters with different origins and ploidy were followed during two years. The results of the study showed intense hemocyte infiltration in gonadal tubules during the ripe stage of gametogenesis in triploid oysters. Finally, this article suggests a link between the frequency of individuals with hemocyte infiltration in gonadal tubules and the proportion of mortality within the triploid groups.