[PROBLEMS] To provide a fat-and-oil in which the content of docosahexaenoic acid is increased.[MEANS FOR SOLVING PROBLEMS] A process for producing highly unsaturated fatty acids comprising culturing a stramenopile capable of producing highly unsaturated fatty acids in a culture medium containing an inhibitor for fatty acid desaturases; fats-and-oils in which the content of highly unsaturated fatty acids, particularly docosahexaenoic acid produced by the relevant method, is increased. A method for enhancing the productivity of highly unsaturated fatty acids in stramenopiles, comprising culturing a stramenopile in a culture medium containing an inhibitor for fatty acid desaturases; stramenopiles having the enhanced productivity of highly unsaturated fatty acids, generated by the relevant method. Particularly, the process for producing highly unsaturated fatty acids, the method for enhancing the productivity of highly unsaturated fatty acids, and the stramenopile having the enhanced productivity of highly unsaturated fatty acids, wherein the stramenopile is a microorganism classified into Labyrinthulea.