Clear differences in bollworm and fall armyworm survival and development were observed when these larvae were fed transgenic Bt cotton leaves from 17 commercially available varieties. A quantification assay (ELISA) was used to quantify the levels of D-endotoxin in two of these varieties (cv. DP 451B/RR & cv. NuCOTN 33B; Delta & Pine Land Co., Scott, MS) throughout the growing season. Differences in the amount of D-endotoxin present in various plant parts and leaves of these two Bt varieties were correlated with larval survival and development. Larvae that were fed on cv. DP 451B/RR completed development faster and exhibited better survivorship than those larvae fed cv. NuCOTN 33B, while lower levels of D-endotoxin were detected in cv. DP 451B/RR compared to cv. NuCOTN 33B.