In this study, interactions on the mortality and debilitating effects between Cry1Ac, a toxic protein produced by Bacillus thuringiensis (Berliner) and HaCPV (Chinese strain) on first and third instars larvae of Helicoverpa armigera were evaluated in laboratory. When first instar was exposed to combination of Bt cotton leaf discs containing HaCPV (6 x 10(6), 1 x 10(7), and 3 x 10(7) PIB ml(-1)) the effect on mortality was additive, when such instar larvae exposed to combination of Cry1Ac (0.9, 2.7, or 8.1 microg g(-1)) and the same concentrations of HaCPV the effect on mortality was additive except for the combination of Cry1Ac (0.3 microg g(-1)) and HaCPV concentrations that showed synergism. When third instars of H. armigera were infected using a suspension containing both HaCPV and Cry1Ac, most combinations of them showed additive effect except for the combination of Cry1Ac (0.3 microg g(-1)) and HaCPV (3 x 10(7) PIB ml(-1)) that showed synergism. However, when they exposed to Bt cotton leaf discs and HaCPV the effect on mortality was synergism except combination of Bt cotton leaf discs and HaCPV (6 x 10(6) PIB ml(-1)) that showed additive. Most of the combinations are showed additive effect in the toxicity and in combinations of Cry1Ac at lowest and HaCPV at highest concentrations synergism is observed. Not only were larval growth and development delayed, but pupation and pupal weight also decreased when larvae were fed on artificial diet containing Cry1Ac and HaCPV or transgenic Bt cotton leaf discs specially in first instar.