Toxicity of two insect growth regulators (IGRs) (lufenuron and hexaflumuron) against two larval instars of cotton leafworm Spodoptera littoralis, laboratory and field strains were determined. Chitinase activity in the two strains also was investigated. Results revealed that, 2^nd instar larvae were more sensitive than 4^th instar larvae to both insecticides. The sensitivity of chitinase activity was measured by I₅₀ values. The I₅₀ values of lufenuron were 0.31, and 0.64 µM for lab and field strains of S. littoralis 2^nd larvae respectively, while I₅₀ values were 0.44, and 0.75 µM for lab and field strains of S. littoralis 4^th larvae respectively. The hexaflumuron were 0.57, and 0.76 µM for lab and field strains of S. littoralis 2^nd larvae respectively, the I₅₀ values were 0.65, and 0.81µM for lab and field strains of S. littoralis 4^th larvae respectively. Also, chitinase enzyme kinetic parameters, as Michaelies-Menten Kinetics (K_m and V_max) values and the inhibition constant (K_i) were determined. The obtained data proved that lufenuron and hexaflumuron compounds are competitive inhibitors of chitinase activity. Results indicated that, the IGRs have shown high potentiality against larvae of S. littoralis, so, these IGRs may be recommended for S. littoralis larvae control, it could be concluded that the use of IGRs instead of conventional hazardous insecticides; may avoid increasing selection pressure of S. littoralis populations to conventional insecticides, hazard effects on human health, environmental components and natural enemies, IGRs may play an important role in future insect pest management programs.