Abstract Oleaginous fungus Mortierella alpina 1S-4 accumulates arachidonic acid (20:4ω6) as a major polyunsaturated fatty acid. However, 20:4ω6 is not essential for the growth of M. alpina 1S-4, because various mutants that produce no 20:4ω6 were isolated. M. alpina JT-180, a Δ12 desaturation-defective mutant that was derived from M. alpina 1S-4 by treatment with a chemical mutagen, accumulates Mead acid (20:3ω9) instead of 20:4ω6. p -Anisidine was found to be a Δ6 desaturation inhibitor in this study. The concentration of 0.1 mg/ml of p -anisidine had an inhibitory effect on the growth of M. alpina JT-180. The addition of p -anisidine to the medium caused M. alpina JT-180 to produce only monoenoic acids such as oleic acid (18:1ω9) and eicosenoic acid (20:1ω9) as unsaturated fatty acids. The effects of exogenous fatty acids were investigated when M. alpina JT-180 was cultivated in medium containing p -anisidine. The addition of linoleic acid (18:2ω6) and 6 Z ,9 Z -octadecadienoic acid (18:2ω9) restored the growth of M. alpina JT-180 cultivated on the medium containing p -anisidine, but palmitic acid (16:0), 18:1ω9, and vaccenic acid (18:1ω7) had no effect. For the growth of oleaginous fungus M. alpina , 18-carbon length fatty acids with more than two double bonds are considered to be essential.