The fatty acid composition of Dictyostelium discoideum has been modified by growing the axenic strain, Ax-2, in media conta-ning long chain polyenoic fatty acids. Large amounts of linoleic and linolenic acids are incorporated into the cellular lipids and further desaturated to two unusual fatty acids, 5,9,12-octadecatrienoic acid and 5,9,12,15-octadecatetraenoic acid, respectively. Arachidonic acid is also extensively incorporated but not further de;aturated. D. discoideum normally contains none of the above polyenoic fatty acids, and the amount incorporated depends upon the concentration of the fatty acid in the growth media. The cells containing large quantities of polyenoic fatty acid grow normally b,t exhibit impaired differentiation when removed from the growth medium. The incorporation of smaller quantities of the fatty acid has no adverse effect on differentiation. Cells grown in the presence of saturated or monoenoic fatty acids exhibit, at the most, only slight changes in the fatty acid composition of the cellular lipid and both grow and differentiate normally.