The present work was carried out under laboratory conditions in order to evaluate resistance building up of a cotton leafworm Spodopte­ra littoralis (Boisd.) laboratory susceptible strain to commercial formula­tions of Deltamethrin, Nuclear Polyhedrosis Virus and their binary mix­tures as indicated by certain biological aspects. The obtained results showed that no resistance have been achieved after 8 successive generations for the three tested treatments. The three formulations displayed latent effect which took place during the larval stages up to moth emergence. The data reveals that larval du­ration, pupal duration, pupal weight, adult longevity, mortality rates, sex ratio and reproductive potentiality were strongly affected. The simulta­neous effect of these tested formulations was greatly governed; the bi­nary mixture demonstrated mild effect in this respect.