Twelve species of marine unicellular algae have been cultured under comparable conditions and the total fatty acids determined by gas–liquid chromatography. Some specific fatty acid relationships paralleling taxonomic groupings were detected but generally individual fatty acids within various algal classes showed large relative variations. The four Bacillariophyceae examined were notable for fatty acid compositions giving iodine values <150, whereas in eight other classes the iodine values were all approximately 200. It is suggested that 5,8,11,14,17-eicosapentaenoic acid, found to be a common algal longer-chain polyunsaturated fatty acid, is characteristically deposited in the lipids of filter-feeders ingesting unicellular algae.