A total of 42 birds from a flock of 104 farmed ostriches showed signs of toxicity after the accidental inclusion of monensin in their concentrate ration. The initial clinical signs were muscle weakness and ataxia which progressed to recumbency, dyspnoea and death, despite intensive supportive therapy. The serum activity of the enzymes creatine kinase, aspartate aminotransferase and lactate dehydrogenase was high in the affected birds, indicating significant muscle pathology. Few gross lesions were identifiable postmortem, but widespread lesions of degenerative myopathy were present at the histopathological level. However, these degenerative changes were restricted to the skeletal muscle and there was no evidence of cardiomyopathy in any of the birds examined. The birds were fed a ration which contained 215 to 224 ppm monensin for 13 days. New clinical cases ceased to occur shortly after the withdrawal of the source of monensin, but all the individuals which showed clinical signs of toxicity died or were euthanased on humane grounds.