Rhynchosporoside, a phytotoxic compound, has been isolated from cultures of Rhynchosporium secalis, the causal agent of scald disease of barley. The toxin is a cello-bioside of 1,2-propanediol. The compound may play some role in symptom expression because it was isolated from diseased plants in concentrations similar to those that could cause symptoms in toxin-treated plants. The toxin causes leaf tip and marginal necrosis and subsequent chlorosis of the entire leaf. The toxin affects only certain cultivars and lines of barley and rye; however, it also affects certain nonhosts of R. secalis. The genetic factor controlling host resistance to the fungus is not identical to that controlling insensitivity to the toxin.