There are many advantages of using microorganisms as practical sources of long chain polyunsaturated fatty acids (PUFAs). After searching a wide variety of microorganisms, fungal mycelia were found to be new and rich sources of C-20 PUFAs of dietary and pharmacologically importance. A soil isolate, Mortierella alpina 1S-4, produced 4.3 g/l (274 mg/g dry mycelia) of arachidonic acid (Ara) on cultivation in a conventional carbohydrate medium. The value accounted for more than 65% of the total fatty acids in the extracted lipids from the mycelia. Eicosapentaenoic acid (EPA) was also produced, when M. alpina was grown at low temperature (i.e., 12℃). This temperature-dependent formation of EPA was found to be due to the activation of the enzyme, which catalyzes desaturation at the n-3 position of Ara, by low temperature. The fungus converted oils containing α-linolenic acid, such as linseed oil and perilla oil, to an oil containing EPA (yield, 1.88 g/l). This conversion was independent of the growth temperature. When grown in the presence of sesame oil, the same fungus produced dihomo-γ-linolenic acid (2.17 g/l) with an accompanying marked decrease in Ara production. This was found to be due to specific inhibition of Δ5 desaturase by sesamin and related lignan compounds preset in the oil. Rat liver microsomal Δ5 desaturase was also specifically inhibited by sesamin. Several unique physiological effects of sesamin to animals are also described.