Laboratory evaluation of eleven Bt cotton cultivars expressing both Cry1Ac and Cry2Ab endotoxins (BT-II) and non Bt cotton on weight of first, second, third and fourth instar larvae of Spodoptera litura. Different plant parts were used i.e. leaves, squares and bolls at 60, 75, 90 and 125 days after sowing of the crop for bioassay. First instar larvae fed on leaves and bolls shows hundred per cent mortality. The final weight of the each instar was reduced at seven days after feeding when compared to non Bt cotton. Reduced in weight was more in case of first, second and third instar than fourth instar larvae. Among leaves, squares and bolls reduction in weight of all four instars was more on leaves followed by squares and bolls