Fatty acids in deep hydrothermal vent bivalves have been analyzed. Their composition is completely different from that of a littoral mussel collected in the Mediterranean sea. The distribution of fatty acids in the littoral mussel is characterized by a predominance of polyunsaturated fatty acids (20:5n-3, 22:6n-3) reflecting the planktonic origin of the food. Vent bivalve fatty acid distribution is dominated by an abundance of the monounsaturated acids (double bond in the n-7 position) 16:1n-7, 18:1n-7, and 20:1n-7 which are clearly of bacterial origin and give an indication of the symbiotic bacterial activity in the bivalves. Differences between the fatty acid composition of the bivalves from two hydrothermal sites (13 degrees N and Galapagos) and differences between the mantle and the gill were observed and are discussed with respect to vent activities at the two sites and species metabolic capacities as a function of ecological conditions.