Cyclopropenoid fatty acids (CPE) isolated fromSterculia foetida oil by urea clathration and reverse phase high performance liquid chromatography (HPLC) were introduced into fungal cultures. Stearate levels in phospholipids and triacylglycerols fromUstilago maydis sporidia rose considerably in response to 30 μM CPE. In addition, CPE themselves were incorporated into glycerolipid fractions. Sterol composition was unaffected. Changes in lipid composition were accompanied by inhibition of dry weight accumulation and sporidial number. Treated sporidia showed irregular wall deposition and a branched morphology. Oleate alleviated CPE effects on growth and morphology. Hyphal extension byRhizoctonia solani was inhibited somewhat by 30 μM sterculate, whileFusarium oxysporum showed no appreciable response. Although CPE appeared to inhibit fatty acid desaturation byF. oxysporum, gross increases in the proportion of stearate were limited to the triacylglycerol fraction during 30 μM treatments. The possibility that the CPE synthesized by plants serve as antifungal agents is discussed.