Bioassays were used to evaluate the effects of plant nitrate levels, diet, instar, and leaf developmental stage on resistance levels in Bt transgenic cotton as shown by mortality of beet armyworm, Spodoptera exigua, when fed leaves from three varieties of Bt cotton plants and their recurrent parents. It was found that high nitrate levels may affect plant expression of Bt resistance and that S. exigua larvae fed on artificial diet as neonates experienced significantly less mortality than those fed exclusively on Bt or recurrent parent leaves. Only first instar S. exigua were consistently susceptible to Bt cotton, though mortality was noted in second instars. Leaf developmental stage had no effect on mortality, but variety may influence resistance levels in Bt cotton.