The effect of chitin synthesis inhibitors when applied singly or in binary mixtures with conventional insecticides against different ages of fourth instar larvae of the cotton leafworm Spodoptera littoral's was studied throughout the present investigation. The results obtained indi­cate that the most suitable age of fourth instar larvae was 24 hr to evaluate the efficiency of chitin synthesis inhibitors. Conventional insec­ticides and their binary mixtures with chitin synthesis inhibitors were more potent to early and middle age of such instar (8-24 hr). All the tested compounds induced lower effect when used against the older lar­vae (40-64 hr). Regarding the effect of chitin synthesis inhibitors on the perfor­mance of insecticides, DC 902 mixture showed an obvious antagonistic effect, while Catabron mixture revealed an additive effect. On other hand, additive and potentiation effects occurred in case of Deenate mix­ture.