chitinase as endo-type of chitinolytic enzymes was studied in the 4th larval instar of the black cutworm, Agrotis ipsilon (Lepidoptera: Noctuidae) to detect some of its physiological aspects. Results showed that, the chitinase activity was low till 18 h before ecdysis to the 4th instar, but the activity started increasing at 10 h and reached a peak at 2 h before the ecdysis. This confirms that the appearance of chitinase activity is restricted to the molting period and the main function of chitinase in larvae is the digestion of old cuticle. On the other hand, the enzyme activity in pupal stage was 1.73-fold as compared to that present in larvae, might to share in the intensive physiological processes in this stage. Results of enzyme localization revealed that larval integument had the highest chitinase activity (72.3%), while hemolymph had the lowest one (1.9%). Hence, we infer that hemolymph plays a minor role in transporting the enzyme, and chitinase localized there in the integument. The results showed that hyperchitinase activity in larvae was not a sign of LC50 treatment by nomolt. This led to the assumption that nomolt acts by inhibition of chitin synthesis not by enhancing of chitin degradation. Future studies concerned with factors affecting chitinase must take into consideration the fact that larval chitinase activity is rhytmic, i.e. precaution that the sampling time is critical.