Acute bovine liver disease, which causes photosensitisation and sudden death in dairy cattle in the southern parts of Australia, has been tentatively associated with the presence of Cynosurus echinatus L. (rough dog’s tail) in the pasture. Also tentatively implicated is the known phytopathogenic fungus Drechslera biseptata that has been isolated from C. echinatus collected at the time of, and from the sites of, outbreaks of acute bovine liver disease. Aqueous and hexane extracts of in vitro cultures of D. biseptata caused binucleation of cultured rat hepatocytes whereas the methanolic extracts caused severe degeneration and necrosis. A methanolic extract of entire C. echinatus collected from a toxic paddock showed the same degenerative toxic effect on the cultured rat hepatocytes. High pressure liquid chromatographic separation and mass spectrometric analysis of the isolated hepatocytotoxic metabolites indicated the possibility of cytochalasin-like compounds.