Field surveys of insect populations in agroecosystems reveal low but significant levels of tolerance to Bacillus thuringiensis(Bt) toxins without mutational changes in resistance alleles. these effects seem to be seperate from target site mutations, providing protection from low to medium toxin does. We have been studying possible mechanisms that provide tolerance tolow to medium levels of Bt toxin does and their implications to overall resistance in field derived laboratory populations of cotton bollworm helicoverpa armigera. the larvae of the moths were exposed in the laboratory to low does of Cry1Ac and Cry2Ab toxins. Although the mechanism initially provide tolerance to low doses of toxins, the level of tolerance is increased by increments if the selection pressure is maintained over subsequent generations. Importantly, for cotton bollworm management, there were several novel outcomes, which may provide a basis for developing improved resistance management strategies to cope with the evolution of new threats to the use of Bt cotton.