The present study aimed to evaluate the biological effect of insect growth regulator flufenoxuron (Cascade) as a chitin synthesis inhibitor against 2^nd and 4^th 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 2^nd and 4^th 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 2^nd instar showed a high sensitivity to the tested IGRs more than 4^th instars. The treated larvae in both 2^nd and 4^th 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.