The fatty acid compositions of sclerotia of Botrytis tulipae, Sclerotinia borealis, and Sclerotinia sclerotiorum produced under specific culture conditions have been compared with those of sclerotia isolated from natural environments. In each organism the lipids of sclerotia isolated from host plants were more unsaturated than those of cultured sclerotia. Fractionation of sclerotial and mycelial lipids of S. sclerotiorum revealed that the neutral lipid component was more unsaturated than the polar lipid component. In cultures of S. sclerotiorum the fatty acid composition of both mycelial and sclerotial lipids was affected by the incubation temperature, becoming more unsaturated as the temperature was lowered, Irrespective of incubation temperature the fatty acid composition of S. sclerotiorum snowed quantitative differences from the "parent" mycelium in that sclerotia consistently contained a greater proportion of oleic acid and correspondingly smaller proportions of palmitic, stearic, and linolenic acid.