The "B. t. product" which is formulated from toxins and spores of Bacillus thuringiensis Berliner as the principal active ingredients, has a highly lethal effect on insects, especially lepidopterous larvae. The lethal effect in general may develop very slowly. It may take several days after ingestion to kill the larvae. On the other hand, the feeding by the larva may be greatly suppressed within a few hours after the ingestion of the B. t, product. In the case of the ingestion of sublethal quantities of the B. t. product, the larva may recover from the loss of appetite when given untreated diet. The writers wish to present the results of a short experiment evaluating quantitatively the feeding suppression by the B. t. product on the larvae of the gypsy moth, Porthetria dispar (L.), and the disappearance of this suppression.