A trehalase inhibitor, validoxylamine A (VAA), gave a lethal effect in Bombyx mori larvae. Overall activities were reduced soon after injection of VAA at the 2nd day of the last instar larva. However, the lethal effect was found at the later stages. NMR analysis showed that trehalose was the main blood sugar and its level was strongly enhanced by the VAA injection. It was supposed that utilization of the energy source was prevented due to the trehalase inhibition, and consequently the overall activities were reduced. But, how the trehalase inhibition caused the lethal effect in the later stages remained unclear.