Our results demonstrated that resistance/lower sensitivity of Mahabubnagar and Parbhani H. armigera larval populations (LC₉₀ values 10.2851 and 15.6230 µg ml^-1) to Bt toxins was due to reduction in proteolytic activity of gut extract results in production of less activated toxin and degradation of the activated toxin to a relatively less toxic product. Resistance of Nanded population (LC₉₀ value 13.7219 µg ml^-1) was due to low amount of the activated toxin produced from the protoxin. In case of Bidar and Kurnool populations, their relative sensitivity to Bt (LC₉₀ values 4.0434 and 5.7445 µg ml^-1, respectively) was due to high binding ability of activated toxin to Brush border membrane vesicle receptor proteins of midgut epithelial cells (137 7 135 µg g^-1).