Sterol and fatty acid composition were combined for comparison of five industrially important Aspergillus species. Quantitative amounts of individual fatty acids, especially unsaturated ones, and the composition of sterols showed the differences among fungi examined. Besides ergosterol, six other sterols were present in detectable amounts in fungal mycelia. The most outstanding were the differences in the amounts of linolenic acid, ergosterol, and lanosterol, as well as in a sterol with M+376 identified as ergosterol peroxide.