The effect of Bacillus thuringiensis toxin Cry1Ac on the behaviour of a laboratory-selected resistant population (72-fold) of Helicoverpa armigera on Bt-cotton was evaluated. Compared with non-Bt-cotton and Cry1Ac toxin incorporated in semi-synthetic diet, resistant larvae reared on Bt-cotton had only 0.13% survival and slower development. The results suggest that Cry1Ac from Bt-cotton exerts a greater toxic effect in terms of larval mortality coupled with decline in larval growth rate compared to semi-synthetic diet.