The quantitative bioassays  were carried out at 80 and 110 DAS by leaf disc feeding method for Helicoverpa armigera (Hub.) and Spodoptera litura (Fab.) using five different Bt cotton event genotypes for characterization resistance to early and late instars. The highest  2nd instar mortality was found 93.1 and 79.2 percent at 80 and 110DAS in Tulasi 4BG-II (MON-15985). The mortality of 3rd and 4th instar was 91.1 and 87.1 percent at 80 DAS respectively for H. armigera. The highest mortality for 2nd instar larvae of S. litura  80.3 and 71.3 per cent at 80 and 110 in Tulasi 4BG-II and the mortality of 3rd and 4th instar was 72.6 and 64.2 was recorded in Tulasi 4BG-II. Whereas Tulasi 4BG-I was recorded least mortality of 2nd instar 7.9 and 5.8 per cent at 80 and 110 DAS. As the larvae gained growth advancement the bioactivity of toxins found to be reduced.