A polyunsaturated fatty acid-producing fungus, Achlya sp. ma-2801, was newly isolated from fresh-water samples obtained in Hiroshima Prefecture, Japan and was investigated with respect to its intracellular lipids and fatty acid composition under various cultural conditions. The eicosapentaenoic acid (EPA) and arachidonic acid (AA) contents reached 32.5 and 61.7 mg/g-dry cell weight, respectively, when grown in a medium consisting of 15 g/l glucose as a carbon source and 7.5 g/l yeast extract as a nitrogen source at 20 degrees C for 6 d. More than 40% of the fatty acids in phosphatidylcholine (78-114 mg/l) and phosphatidylethanolamine (33-39 mg/l) comprised EPA and AA when Achlya sp. ma-2801 was cultivated at 15 degrees C.