The present study aimed to evaluate the biological effect of insect growthregulator flufenoxuron (Cascade) as a chitin synthesis inhibitor against 2nd and 4th larval instars of Spodoptera littoralis, to determine its toxicity. Effect of sublethal doses LC25, LC50 and LC90 were used to investigate the enzymatic activities. The tested IGR significantly increased the larval and pupal durations, on the other hand decrease the percentages of pupation, adult emergency, fecundity and fertility of the eggs produced by the adult progeny. The tested compound significantly induced larval mortalities, which were dose dependant. Treatments of the 2nd and 4th larval instars with the tested IGR induced some morphogenic abnormalities in larval, larval-pupal and pupal stages, as well as pupal-adult intermediate. Some emerged adults have various degrees of malformations. All the treated larvae as 2nd instar showed a high sensitivity to the tested IGRs more than 4th instars. The treated larvae in both 2nd and 4th larval instars with the sublethal doses LC25, LC50 and LC90 showed a significant decrease in enzyme activities of acid phosphatase and the non- specific esterases, α,β esterases at different times intervals post treatments.