A dermatitis characterised by discrete, raised, plaque-like and cracked skin lesions of variable sizes on the udder, the hind quarters, the lips and muzzle of all the cows in a herd was suspected of being caused by the oat straw used in bedding, after initial feed analysis and skin culture were negative for toxins and dermatophytes. Mycological analysis indicated an extensive infestation of the oat straw by Fusarium sporotrichioides, a toxic mould, and an immunochemical assay indicated dermatotoxic trichothecenes in the straw (0.22 μg/g dried straw). An ethyl acetate extract of the straw induced a necrotic response on shaved rat skin. Ingestion of the toxic bedding straw and inhalation of toxic straw dust probably also caused the internal haemorrhage and lung emphysema observed in the two cows that died. The regression of the dermatitis and the recovery of general herd health after the withdrawal of the oat straw further supported the diagnosis.