The relative pathogenicity and toxicity of nuclear polyhedrosis virus of the cotton leafworm, Spodoptera litioralis (Boisd.) (SLNPV) and three pesticides, namely DC–702, DC–703 and DC–800, were investigated against the second- and fourth-instar larvae of S. littoralis. Results revealed that the second instar was more susceptible to the action of all treatments than the fourth instar. Among the tested insecticides, DC–703 was most toxic for either instars, followed by DC–800 and DC–702 for the second instar larvae. Fourth-instar larvae were more susceptible to DC–702 and DC–800, respectively. Combination of virus and DC–702 gave increased larval mortality when compared with the effects of either alone. On the other hand, antagonistic effect occurred when DC–703 was combined with NPV. This may be due to the high percentage of diflubenzuron (4.8%) as one of the components of DC–703, while DC-702 contains only 2.4% diflubenzuron. Synergistic interactions of mixtures containing low concentrations of SLNPV and DC–702 or DC–800 may be effective against S. littoralis and more safe for other biotic control agents.