During the course of searching for biologically active natural products from fungi that possess plant growth regulatory activity, we isolated Botrytis cinerea from stored sunflower seed (Helianthus annuus L.). Mass culture of the organism and subsequent extraction of the fermentation medium led to the isolation of a novel metabolite, trivially named cinereain. We now report the isolation, identification, and biological activity of this metabolite. Cinereain is a ruby red crystalline product with the molecular formula C18H21N3O3 and a melting point of 201-203°C. Its structure was unequivocally established by single crystal X-ray diffraction, with UV, IR, 1H/13C NMR, and mass spectrometry supporting the crystallographic data. Cinereain significantly inhibited the growth of etiolated wheat coleoptiles (p<0.01) at concentrations of 10^-3 and 10^-4 M in bioassays. No apparent effects were induced in greenhouse-grown bean or tobacco plants; however, slight necrosis and mild chlorosis were observed in corn five days after treatment with 10^-2 M cinereain, and these plants recovered within two weeks. No effects were noted in disk assays against Bacillus subtilis, B. cereus, Mycobacterium thermosphactum (all Gram-positive), Escherichia coli, or Citrobacter freundii (Gram-negative) bacteria.