- Abstract Polyunsaturated fatty acids (PUFAs) exogenously supplied to the red microalga Porphyridium cruentum were incorporated into cellular lipids. All the C 18 PUFAs studied were desaturated by a Δ 6-desaturase and all the C 20 PUFAs by a Δ 5-desaturase. The latter enzyme desaturated even 20: 2 (11, 14) to 20: 3 (5, 11, 14) and 20: 3 (11, 14, 17) to 20: 4 (5, 11, 14, 17). We infer the existence of several fatty acid desaturases, with different chain length specificities. Furthermore, the introduction of double bonds in a methylene interrupted pattern, at least for α-type desaturase such as the Δ 5-and Δ 6-desaturases, is not an intrinsic property of the enzyme but a consequence of the arrangement of the pre-existing double bonds in the fatty acid substrate.