The antifeedant and antimolting effects of azadirachtins A and B, and of a synthetic derivative, 7-acetyl-azadirachtin A were examined. Given through a blood meal, the effective dose (ED 50 ) for the antifeedant effect ranged from 25 to 30 μg/ml of blood for the three compounds. ATP, a phagostimulant, also given orally, reversed the antifeedant action of azadirachtin A. The ED 50 values for molt inhibition were 0.04, 0.015, and 0.45 (μg/ml of blood for azadirachtins A, B, and 7-acetyl-azadirachtin A, respectively. The mechanisms of feeding and molt inhibitions by azadirachtin A B, and its 7-acetyl-azadirachtin A derivative are discussed.