Validoxylamine A showed a potent inhibitory activity against insect trehalase in a competitive manner with a Ki value of 4.3 x 10(-10) M. The other validoxylamines and validamycins also exhibited the activity in vitro. Injection of these compounds to young last instar larvae of the tobacco cutworm, Spodoptera litura, elicited the morphological abnormality followed by death after the cessation of feeding. Validoxylamine A showed 100% mortality at a dose of 10 micrograms/ last instar larva.