An outbreak of acute myopathy occurred in a group of 23 Hereford cross and Aberdeen Angus cross 18-month-old heifers, running outside with a bull in February. One morning two were found recumbent, five were ataxic and one had fallen into a river and drowned. Subsequently two of the ataxic ones became recumbent. Serum creatine phosphokinase values were found to be directly proportional to the severity of the clinical manifestations. Myoglobinuria was not observed. Serum calcium, magnesium and inorganic phosphate levels were all within the normal ranges. Treatment of affected animals with selenium and/or alpha-tocopherol had no clear effect; from biochemical estimations carried out, the selenium and alpha-tocopherol status of both affected and unaffected animals was considered adequate. Five recovered, two spontaneously without treatment, but the two most severely affected did not improve with therapy and had to be slaughtered 13 and 25 days respectively after the onset of symptoms.