Attempts were made to investigate on the fatty acids of Agaricus bisporus, a cultivated edible mushroom. Fats were extracted from it and saponified with alcoholic potassium hydroxide. Isolated fatty acids were methylated and were subjected to column chromatography and G.L.C. Eleven saturated fatty acids, i.e., dodecanoic, tridecanoic, tetradecanoic, pentadecanoic, hexadecanoic, heptadecanoic, octadecanoic, eicosenoic, uncosenoic, docosenoic, and tricosenoic acids, were identified. Especially palmitic and stearic acids were abundant. Nine unsaturated fatty acids appear to exist in the carpophore.