Culture filtrates of several strains of Fusarium equiseti (Corda) Sacc. were found to be highly phytotoxic. Several phytotoxic substances were isolated; the most important of these was a colourless crystalline compound, diacetoxyscirpenol, m.p. 161-2°, C19H26O7. Though highly phytotoxic in many respects, it markedly stimulates elongation of cress roots at 0.01–0.5 μg./ml. This is not an anti-auxin effect. Diacetoxyscirpenol is at most only slightly inhibitory to fungi and bacteria, but is highly toxic to rats. Two minor metabolic products, m.p. 135-6° and 185-8° respectively, are also described. They, too, were highly phytotoxic, though their spectra of activity were somewhat different from that of diacetoxyscirpenol. They also were highly toxic to rats. Production of these substances is probably not important in the aetiology of disease symptoms in plants parasitized by F. equiseti, but may be significant in cases of toxicosis of animals fed with grain infected by Fusaria. Scirpentriol, a hydrolysis product of diacetoxyscirpenol, is much less phytotoxic but of equal toxicity to rats.