Abstract: Effects of deltamethrin on the nemo lymph esterases activity (HEA) of the 5th instar larvae of Dendrolimus punctatus were studied. For the untreated larvae, both the summer larvae and the larvae after hibernation have a∽-form curve in the daily dynamic changes of HEA. The results also showed that the daily dynamic changes of HEA were negatively correlated with those of hemolymph lipid content(HLC), and positively with hemolymph carbohydrate content(HCC). The treatment of sublethal doses (LD5) of deltamethrin could cause a positive correlatoin between HEA and HLC in both susceptible strain and tolerant strain, and a negative correlat on between HEA and HCC in the tolerant strain. The activity of hemolymph esterases was found to be closely related to the poisoning syndrome of the deltamethrin. For the susceptible strain, the HEA depleted at the knocking down and conveulsive stages, elevated at exiting and recovering or dying stages. Whereas in the tolerant strain, the HEA depleted at the knocking down stage only, and elevated gradually than after. In those two strains, the levels of HEA in the recovering and dying larvae were the same, this indicated that the hemolymph esterases play a less important part in the lethal effects caused by deltamethrin. Finally the importance of hemolymph esterases in the metabolism of hemolymph lipid and carbohydrate were dicussed.