The present work studied the effect of the organophosphate insecticide, sumithion on the liver of toads Bufo tibamicus. Feeding toads with a daily dose of sumithion (40 mg/kg body weight) for 10 days caused histological alterations in the liver. The normal structural organization of the hepatic acini was impaired, the hepatocytes showed cytoplasmic vacuolation and the blood vessels were congested. Moreover, elevations of transaminases (GOT, GPT) were recorded in the sera of treated animals. The obtained results collectively indicated that sumithion treatment induced liver injury in the toads.