A phytotoxic compound was isolated from a liquid culture medium of Stemphylium botryosum, a pathogen of lettuce. The toxin is an amorphous yellow solid with absorbance maxima at 218, 268, and 427.5 nm and exhibits a bathochromic shift in alkaline pH. It has a molecular weight of 370 and an empirical formula of C(17)H(22)O(9). Glucose and aromatic pigments are detected after acid hydrolysis. Based on its spectral and chemical properties, the proposed structure of the toxin is 3-hydroxy- 2,2-dimethyl-5-alpha-d-glucopyranoside-2,3-dihydrochromone, and it has been given the trivial name stemphylin. A linear relationship exists between lesion area and amount of toxin applied to a young lettuce leaf. The relationship between toxin production and the development of disease symptoms is discussed.