Parasiticol, a metabolite structurally related to the aflatoxins, has been isolated and characterized. The compound is elaborated by several strains of the Aspergillus flavus series previously shown to be aflatoxin producers. The metabolite was separated and purified by chromatography on three columns (silicic acid, silica gel, and alumina) followed by recrystallization from chloroform-hexane. Parasiticol is differentiated from aflatoxins B1, B2, G1, G2, M1, M2, B2a, G2a and aspertoxin by thin-layer chromatography. Ultraviolet and infrared absorption spectra, nuclear magnetic resonance, and mass spectrometry studies were used to elucidate the structure. Parasiticol is as acutely toxic to ducklings as aflatoxin B1, but it is only 1/100 as toxic as B1 in chick embryo studies.