The present study aimed to the evaluate the effect of two Chitin Synthesis Inhibitors, Lufenuron (Kafroseil^® 5%) and Chlorfluazuron (Caprice^® 5%) on 2^nd and 4^th instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.). Results showed that the 2^nd instar larvae were more susceptible than 4^th instar larvae, as denoted by the determined low LC₅₀ obtained for 2^nd instar larvae. Results also showed that larval and pupal duration, and pupation and the adult emergence percentage, of survived larvae were significantly decreased compared to control. Moreover, adult longevity for male and female moths were also affected by treatment. In addition, adult fecundity and fertility of survived-treatment larvae was also reduced. Results also revealed that treatment with tested compounds has dramatically affected certain enzyme activities.