AbstractThe ciliate,Tetrahymena, was provided a supplement of the fatty acid [1‐14C] 18∶2Δ6,9. After a period of growth the cells were claimed, the lipids extracted, the polar lipids recovered and the mild alkali‐labile fatty acid methyl esters generated. The fatty acids were resolved by high pressure liquid chromatography (HPLC), the 18∶3Δ6,9,12 (γ‐linolenic acid) was recovered and its identity verified by high pressure liquid chromatography (HPLC), gas liquid chromatography (GLC), hydrogenation and oxidation. Fifty‐three percent of the cell‐associated label was found in γ‐linolenic acid; thus, a Δ12 fatty acid desaturase converts the 6,9 octadecadienoic acid to the 6,9,12 derivative. No carboxyl or methyl terminus restriction appears on Δ9 monoenoic or dienoic fatty acid desaturation in this cell as is found in higher plants and animals.