Abstract: The present study elaborates on the results of cross-resistance of Cry1Ac resistant Helicoverpa armigera (Hübner) to spore-δ-endotoxin complex of Bt strains (HD-1, HD-4, HD-8, HD-57, HD-133, HD-234, HD-263, HD-282, HD-551, HD-744 and Bt-based biopesticide. In this study, all the bioassays were done on larvae of 55 th, 0 th and 61st Cry1Ac selected generations of H. armigera. The resistance ratios (RR) among Cry1Ac selected generations varied from 172 to 417 fold in different generations. Cry1Ac selection exhibited low resistance ratios (2.722, .756) against HD-1 and HD-8 among all the HD strains tested and the slopes for HD-1 and HD-8 were highly significant (1.49 and 1.31, respectively). The test insect also showed low resistance ratios 3.959 and 4.385 for HD-551 and HD-57 respectively with significant slopes 1.18 and 2.36 accordingly. The spore-crystal toxins of strains of HD-1, HD-8, HD-263 and Bt-based biopesticide caused significant reduction in larval weight (%) with regressions y = 0.011x+93.912, R2 = 0.7269, df = 4, p<0.05 and y = 0.03x+85.332, R2 = 0.6982, df = 4, p<0.05, y = 0.0458x+79.58, R2 = 0.7271, df = 4, p<0.05 and y = 0.0776x+59.169, R2 = 0.8273, df = 4, p<0.05, respectively against Cry1Ac selection. Significant slopes 1.49, .31, .36, .67, .18, .12 and 2.09 for HD-1, HD-8, HD-57, HD-263, HD-551, HD-744 and Bt-based biopesticide, respectively indicated fast interaction between spore-crystal toxins of Bt strains and the Cry1Ac resistant insect. Similarly for susceptible H. armigera, significant regression trends in term of percent larval weight reduction were observed for HD-263 (equation y = 0.0374x+82.173, df = 4, p<0.05, R2 = 0.7098), HD-1 (y = 0.0501x+76.35, df = 4 p<0.05), HD-8 (y = 0.0584x+68.508, df = 4, p<0.05) and Bt-based biopesticide (y = 0.0968x+57.063, df = 4, p<0.05). Previous studies conducted by other workers indicated synergistic impact between spores and crystals of Bt strains against resistant insects. Present results suggest that the spore-crystal complex (perhaps due to synergistic interaction between or among the spore and crystal components) may have caused synergism due to which a Cry1Ac selection was knocked down. Thus, spore-crystal complex-based formulations of Bts can be developed for use as an effective tool to manage insect populations that have development resistance to single toxin (Cry1Ac) produced in transgenic crops (e.g., Bt-Cotton).