Internal defence mechanisms of bivalves include the hemocytes which assume several functions. Relationships exist between environmental factors, physiological state of oysters, defence system and pathogens. Hatchery oysters are more and more produced and reared in the field. The characteristics and particularly the defence system of these animals are poorly documented. In this context, two field trials were carried out from May to September 2002 and from April to July 2003 in order to monitor hemocyte parameters of diploid oysters from natural beds and hatchery-produced diploid oysters using flow cytometry. Several hemocyte parameters (cell mortality, granulocyte percentage, phagocytosis, esterase and peroxidase activities) were monitored. For both oyster groups, trend of hemocyte parameters in 2002 and 2003 are reported. For several parameters, values tended to increase over the sampling period in both experiments. For both experiments, phagocytosis was higher for diploids from natural beds compared to hatchery diploids. This is the first study in which trend of hemocyte parameters from the Pacific oyster, Crassostrea gigas, was monitored with short sampling intervals and to compare hatchery diploids and diploids from natural beds.