During the last two decades, knowledge of shellfish immunology has been largely improved and the immune status of bivalves can be assessed by the measurement of both cellular and humoral parameters. Previous monitoring studies which the immune status of bivalve molluscs has been assessed have demonstrated that many parameters may vary greatly among sites and seasons, suggesting that environmental and endogenous factors may affect this system. Reproduction may also interfere with the immune system. This study focuses on the seasonal variations of the immune parameters in two batches of Pacific oysters differing by their ploidy, diploid′s and triploid′s. The oysters were sampled from a French oyster farm over two reproductive periods. Selected immune parameters investigating haemocyte integrity and immunocompetency were analysed monthly by flow cytometry. Histological observations were made in gonads to discriminate gender and gametogenesis stages. Results indicate that immune parameters in both diploid and triploid oysters exhibited marked seasonal variations. Phagocytosis index was significantly low during spawning before reaching a maximum in autumn. Triploids also appeared to be less sensitive to environmental cues than diploids. Taken as a whole, alterations observed in maturing individuals corresponded to a depressed immune status in late spring, at a time corresponding to the spawning period. Furthermore, grouping by sex of ongoing gametogenesis individuals revealed uneven immune alterations among males and females. Finally, grouping sexual maturation stages showed evidence of a peculiar stressful event during early gametogenesis, perhaps under endogenous control.