The biochemistry of amebas is an essentially unexplored field. What studies there have been are almost exclusively concerned with Amoeba proteus and Amoeba chaos chaos, neither of which is optimally suited to most biochemical investigations for they can be grown only in relatively small quantity and must feed on another organism. Acanthamoebu sp., a soil ameba, was isolated by Neff (1) and cultured in the axenic state on a soluble medium of proteose-peptone and glucose. Subsequently, it was found by Adam (2) that Acanthamoebu will grow on a chemically defined medium containing 18 amino acids and several vitamins. Since the organism can be grown in submersion culture (3), there is no limit to the quantity of cells that can be obtained. Acunthumoeba, therefore, would seem to be ideally suited to any type of biochemical study. The amebas can be handled experimentally like, and offer many of the advantages of, other microbial organisms. Because they are animal cells, it may be anticipated that thorough study of amebas will be of particular utility in furthering our understanding of those aspects of mammalian biochemistry that appear to be unique to animals. One such area is the metabolism and function of certain polyunsaturated fatty acids. This paper reports the composition of the fatty acids of Acunthamoeba. It has been found that the organism contains a particularly high concentration of the unsaturated fatty acids 20:1(11),’ 20:2(11,14), 20:3(8,11,14), and 20:4(5,8,11,14). Thus, Acanthamoebu would seem a useful organism for the study of the biosynthesis of these acids, and, hopefully, of their function, an apparently essential one in higher animals.