Glutathione S-transferase activity was determined in the lepidopteran insect species,Achaea janata, during larval, pupal and adult stages following treatment with sublethal and lethal doses of fenitrothion. Both doses of insecticide produced significant induction of enzyme activity. The rate of induction of enzyme activity was not significantly different in insects that received sublethal and lethal doses of insecticide. Enzyme activity in the different stages of insecticide-treated insects was in the order pupa > adult > larva. However, the inducing effect of the insecticide was higher in larvae, than in pupae and adult. In the absence of induction, the level of enzyme was as much as 3 times higher in midgut tissue than in carcass. In larvae treated with sodium barbitone along with fenitrothion, the knock-down effect of the insecticide was delayed. This was attributed to the increased induction of glutathione S-transferase in the larvae treated with sodium barbitone. The level of reduced glutathione, a rate-limiting factor in the induction of glutathione S-transferase, changed in a cyclic manner in insecticide-treated larvae.