The localization of the midgut esterase in the cabbage worm, Pieris rapae, during the last larval period was observed with cytochemical treatment using transmission electron microscope and esterase activity was also identified by electrophoresis. The reaction product of esterase to the substrate solution (mixture of a-naphthyl acetate and fast blue B salt) was observed both in the protoplasmic processes of the goblet cells and in the microvilli of the columnar epithelial cells as electron dense granules. Twelve esterase bands were separated in the midgut during this period and the major midgut esterase was identified to be the carboxylesterases.