Gametogenesis in the giant scallop,Placopecten magellanicus(Gmelin), occurs in early summer and spawning takes place during August and September. The gonad differentiates between March and May, when the water temperature is low (0 to −1 °C) and food supply high. Gonad growth also begins at this time. Energy reserves from the previous year do not seem to play a significant part in subsequent growth and maturation of the gonad. There is no accumulation of metabolic reserves in the scallop gonad after spawning; in winter, the gonad is very small and inactive. Levels of lipid (mostly phospholipids and free sterols), protein, low molecular weight carbohydrate, and glycogen increase in the plasma during the period of gonad growth and maturation. The amount of glycogen in the circulating hemocytes is greater in summer than in winter.Key words:Placopecten magellanicus, bivalve, scallop, reproductive cycle, blood, hemolymph, hemocyte, blood chemistry, glycogen, phospholipid, sterol