Oral administration of glibenclamide, the new antidiabetic drug, caused hypoglycemia in Clarias batrachus. The effect was maximal at 15 hours, when some fish had practically no sugar in their plasma. The hypoglycemic effect was insignificant in partially-pancreatectomized fish, indicating that the drug potentiated the effect of insulin in such animals. In fasting normal fish, the drug provoked an increase in the liver glycogen contents while in the diabetic animals, a reverse condition occurred. Significant beta-cell degranulation and vacuolisation was noted in the Langerhans' islets, the effect being synchronous with hypoglycemia. In a few cases, the alpha-cells had also undergone degenerative changes. No significant side effects and discerneable toxicity was observed in the treated-fish. It is stipulated that the drug is endowed with the beta cytotropic property.