Stachybotrys alternans produces a series of low-molecular-mass compounds. By their column chromatography we have isolated the nitrogen-containing compound stachybotrin. The structure of stachybotrin has been determined on the basis of chemical transformations together with the results of 1H and 13C NMR spectroscopy, 2D NMR 1H-1H and 1H-13C chemical shift correlation (COSY), correlation of long-range 1H-13C interactions (HMBC), measurements of NOEs in a rotating coordinate system (ROESY), and IR and electron-impact mass spectra. Stachybotrys alternans damages cellulose-containing fodders, producing mycotoxins causing toxicoses in animals [1]. The chemical nature of these mycotoxins has not been elucidated. In view of this, we have studied the low-molecular-mass metabolites of Stachybotrys alternans. By column chromatography of the total products of the vital activity of the organism grown under laboratory conditions we isolated the quantitatively predominant component, which we have called stachybotrin (1, Scheme 1). In the present paper we report the establishment of the structure of this compound.