Summer mortality is an important economic concern for the Pacific oyster, Crassostrea gigas, industry all over the world and particularly in France. Outbreaks appear when oysters are in gonadal maturation. Bivalve defence mechanisms against pathogen invasion are assumed by circulating cells, the hemocytes. A field experiment was conducted in the Marennes-Oleron Basin (Charente-Maritime, France) in order to monitor hemocyte parameters, gametogenesis and mortality rates among C. gigas during 7 months from March to September 2002. Diploid (from natural bed and from hatchery) and triploid oysters were placed at 15 cm or 70 cm above sediment at Marennes-Oleron Basin. Triploid animals were included because they are considered as sterile animals. Mortality rates were monitored and lipid contents were analysed in order to define sexual maturation stages. Hemocyte parameters (cell mortality, enzymatic activities and phagocytosis) for the three oyster groups were monitored by flow cytometry. Results were analysed by pooling data for the whole sampling period and by separating data in three periods related to the gametogenesis process (A, B and C). Mortality outbreak occurred during the ripe gamete period (B). Results showed that triploids presented the highest values for several hemocyte parameters and the lowest mortality rates. Differences between triploids and diploids were highest during the ripe gamete period. Relationships between oyster mortality, gonadal maturation and hemocyte parameters are discussed. This is the first study monitoring hemocyte parameters of wild diploid, hatchery bred diploid and triploid oysters reared in the field. (c) 2005 Elsevier B.V. All rights reserved.