No morphological difference was found in the epithelial cells of midgut between the normal and the diflubenzuron-treated last instar larvae of the cabbage armyworm, Mamestra brassicae L. Diflubenzuron and polyoxin D strongly inhibited chitin synthesis in the peritrophic membranes of the midgut when glucosamine or N-acetylglucosamine was applied, as a precursor of chitin, either inside or outside the midgut. When UDP-N-acetylglucosamine was injected into the midgut with polyoxin D, chitin synthesis was blocked, but not with diflubenzuron. The results lead to the following speculation: UDP-N-acetylglucosamine would be formed from glucosamine or N-acetylglucosamine in the epithelial cells and then move to the outer face of the biomembranes of the cells to form chitin. Diflubenzuron may act as an inhibitor of the transport system of UDP-N-acetylglucosamine across biomembranes, since an accumulation of UDP-N-acetylglucosamine is known in insects treated with diflubenzuron. Na⁺-K⁺ ATPase and Ca²⁺-Mg²⁺ ATPase were not affected by the insecticide.