In the bay of Marennes-Oléron the dry flesh of Crassostrea gigas ranged from 29 to 52% proteins, 10 to 23% ash, 6 to 20% lipids and 0 to 4% carbohydrates of which 48.3 ± 21.6% were glycogen. The energy value ranged from 18.84 to 23.47 joule.mg−1 ash free dry weight (mean 20.27 J.mg−1). Reproductive effort varied from 17.8 to 55.9% dry weight for males, and 43.1 to 61.9% for females during the first and third year of reproduction respectively. A high energy value for female gametes (23.47 to 26.24 J.mg−1 dry tissue) was mainly due to a higher level of lipids (17.3 to 23% of dry tissue) in female gametes as compared to that in male gametes (5.9 to 12.3% of dry tissue). During the period of mass-loss which began in October and finished in March, the poor trophic conditions of the environment resulted in the use of 74.6% of glycogen, 44% of lipids and 36% of proteins to meet more than 50% of the requirements for basal metabolism.