An extract of rhizomes of Curcuma longta L. (turmeric) inhibited the desaturation of dihomo-gamma-linolenic acid (DGLA) in the arachidonic acid (AA) producing fungus Mortierella alpina 1S-4. The factor responsible for this phenomenon was isolated and identified as curcumin (diferuloyl methane). Mycelial DGLA levels increased about two-fold (22.3 mg/g dry weight) with a concomitant decrease in AA levels when the fungus was cultivated with curcumin. The 50% inhibitory concentration against delta 5 desaturase was 27.2 microM. Curcumin also inhibited rat liver microsomal delta 5 and delta 6 desaturases.