Haemolyph carboxylesterases induced in diapausing pupae of Helicoverpa assulta Guenee were investigated. Increase in the activity of the electrophoresed isozyme bands were observed during the diapausing pupae. The isozymatic composition exhibited remarkable alterations represented as disappearance and induction of some isozyme bandsp which were identified as carboxylesterase (CE) on the basis of their specificities to inhibitors. Much higher activity of the induced CE was shown in reaction with $eta$-naphthyl acetate ($eta$-Na) than $alpha$-naphthyl butyrate ($alpha$-Nb), representing the high regioselectivity to $eta$-naphthyl group. Optimal temperature for the enzyme activity was different to the substrates used 37$^{circ}C$ in $eta$-Na and 4$0^{circ}C$ in $alpha$-Nb, respectively. However, the optimal pH for the enzyme activity was the same as 7.5 regardless of the substrates used, and relatively high thermostability of the CE was demonstrated by showing the denaturation at high temperature (50~55$^{circ}C$).