Bioassay was carried out  for monitoring resistance spectrum toward insecticides include organophosphates, Dursban H-48% EC (Chlorpyrifos)  and pyrethroids Cyper (Cypermethrin 10% EC) in three  different field populations (Gharbia , Kalubia and Menofeya Governorates) of the Cotton Leafworm, Spodoptera littoralis (Boisd.). Toxicity data based on LC₅₀ values indicated that the pyrethroid Cyper (Cypermethrin 10% EC)  is more  toxic insecticide against the  laboratory strain, while,  the OP Dursban H- 48% EC (Chlorpyrifos) was less toxic. The development of resistance to organophosphates, Dursban H-  48% EC (Chlorpyrifos)  and  pyrethroids Cyper (Cypermethrin 10% EC) in field strains of the cotton leafworm, in relation to esterases, glutathione s- transferase, and phosphatases was studied. Field strains exhibited moderately high level of resistance to the tested insecticides. A significant increase in the activity of esterases, glutathione s- transferase suggests a good relation to resistance was detected. A high level of acid phosphatase and low level of alkaline phosphatase accompanied the level of resistance to pyrethroid and OP.