The fatty acid composition of particulate matter from surface waters in the North Atlantic has been investigated. A high proportion is long-chain polyunsaturated acids, typical of a marine origin. The low relative abundance of iso and anteiso 15-carbon acids indicates little bacterial contribution to the particulate matter. This fatty acid distribution is in contrast to that found in estuarine particulate matter, which contains only trace amounts of the long-chain polyunsaturated acids and substantial quantities of branched chain acids.