In order to investigate the seasonal variations of carbohydrate-metabolizing enzyme activities and body compositions in the adductor muscle and internal organ of Japanese pearl oyster Pinctada fucata martensii, the bivalve were cultured from May 1996 to July 1997. The glycogen content in adductor muscle varied seasonally with the accumulative quantity of phytopigment concentration in environmental water, and decreased with increases in the activities of phosphorylase, phosphofructokinase, and pyruvate kinase in the tissue from September to October. Thereafter, the glycogen content reached a poor level in November, when the activities of their glycolytic enzymes decreased remarkably. The activity of glucose-6-phosphate dehydrogenase in internal organ coincided seasonally with reproductive cycle and the absolute quantity of crude fat in this tissue, suggesting the provision of NADPH for the synthesis of fatty acids and sterols. These results suggested that the seasonal variations in carbohydrate-metabolizing enzyme activity and body composition of pearl oyster are influenced mainly by feed quantity and sexual maturity.