Variations in fat content, glycogen and calorific values for meat content were studied inCrassostrea virginica (Gmelin) from Mississippi Sound with reference to sex and size during four different seasons of the year. Lean (October) oysters were characterised by lower glycogen, greater percentage of hydrocarbons, and sterols. Fat (February) oysters were characterised by higher glycogen, phospholipid, and triglyceride. There were size- and sex-related variations in the calorie content of the meat of oyster in all the months tested. In April the oysters had more calories. The free sterol fraction of oyster fat, yielded the highest calorific energies. The phospholipids from the lean oyster yielded more calories than those from fat oysters. The degree of unsaturation of the lipid class was believed to cause these variations in the calorie content of oysters.