Extracts of cultures of Aspergillus fumigatus isolated from silage were lethal to chicken embryos. Using this test and thin-layer chromatography, four UV-absorbing toxins, designated as fumitoxins A, B, C and D, were isolated. Analysis and mass spectrometry of crystallized fumitoxin A, the most abundant in the extract, established its molecular formula to be C31H42O8. Infrared, UV spectroscopy, and chemical reactions suggested that fumitoxin A is a steroid. Fumitoxins appear to be clearly different from the previously described toxins recognized in A. fumigatus.