A new disaccharidase inhibitor, AO-128, showed 190-3900-fold more potent inhibition of purified rat small intestine sucrase-isomaltase (S-1) complex and 23-33-fold more potent inhibition of semipurified porcine small intestine disaccharidases than acarbose. AO-128 suppressed elevation of the blood glucose concentration after oral sucrose, maltose, and starch, but not after oral glucose, fructose, and lactose. The chronic addition of AO-128 to the diet produced antiobesity and antidiabetic actions in obese and/or diabetic animals. Undesirable side effects, such as diarrhea and soft feces, were observed only for the first 5-7 d and suppression of intestinal disaccharidase activities was observed even at the end of the experiment, suggesting that the suppressive or delaying effect of AO-128 on elevation of the postprandial blood glucose concentrations is involved in reduction in body weight gain and prevention and/or amelioration of the diabetic state. Thus, AO-128 is useful as an adjunct to the dietary management of obesity and diabetes.