SYNOPSIS. The American oyster, Crassostrea virginica, has some metabolic traits typical of marine bivalve molluscs, andsome that are apparently unique. Cvconsumption is rapid when the valves are open, but may become and remain zero for days on closure. Rates of nitrogen-excretion are similar to those of other marine bivalves; the chief end-products are ammonia (65 –70%) and amino acids (5–21 %), but small amounts of urea and uric acid are regularly found. Deoxyribonucleic acid (DNA) of marine molluscs has a low content of the base-pair, guanine-cytosine (GC); oyster DN'A has thelowest GC (about 28 %) of nine species studied. There are six sterols, the major one cholesterol, and there is evidence of changes in steroid metabolism with the gonadal cycle. Aminotransferase activities of marine bivalves are proportional to rates of loss of amino acids, suggesting that the enzymes function to assure continuous replenishment. The oyster isunique in its extraordinarily low rate of conversion of alanine to pyruvate. Most of the intermediates and enzymes requiredfor complete oxidation of glucose have been found in some species of Crassostrea, but anaerobic glycolysis produces more succinic than lactic acid. A partial explanation lies in ready reversibility of succinate and laclate oxidoreductases.