Variations in biochemical composition and lipid classes in relations to the gametogenic cycle were studied in digestive gland, labial palps and gonad in the Pacific oyster Crassostrea gigas from the Western Atlantic coast of France over 1 y. Variations in the weight of the soft body parts, gonad development and spawning depended on environmental conditions, mainly on nutrients availability. The main energy reserve is the glycogen, showing a marked seasonal variability in all tissues studied this component increased in the digestive gland during the phytoplankton blooms and was rapidly transferred and accumulated in labial palps and gonad during stages IIID-0; glycogen was then mobilized during the final stage of somatic growth and during the beginning of the gametogenic cycle (stage IA). Glycogen is consumed almost totally at the beginning of spring, coinciding with an important increase of phospholipids in males and triacylglycerols in females. This fact seems to indicate that the glycogen reserves were used in the synthesis of lipids and in supporting the high energy demand existing during the active vitellogenesis and spermatogenesis. The digestive gland, besides playing an intermediary role in transferring the ingested lipids to other organs, seems to constitute an important organ of reserve of lipids, mobilizing phospholipids in winter towards palps and gonad during the somatic growth, but also during the initial gametogenesis and sexual maturation (triacylglicerols). There is depletion in the reserves of glycogen and triacylglicerols of the labial palps during the last stage of gonadal maturation. Simultaneously, an increase in triacylglicerols in gonad occurs, suggesting that the labial palps were an organ of reserve to sustain the most advanced stages of gonadal ripeness. The changes of sterol esters of gonad are discussed with regard to gametogenic cycle of Crassostrea gigas.