Bollworm, Helicoverpa zea (Boddie), and tobacco budworm, Heliothis virescens (F.), injury to transgenic cotton expressing the delta-endotoxin from Bacillus thuringiensis var. kurstakii, and non-transgenic cotton was sampled over the growing season at College Station, Texas in 1990 and 1991. Dramatic differences were found in insect injury to flower buds and bolls of transgenic cotton compared with non-transgenic. Higher percentages of injury were observed on non-transgenic cotton than on transgenic cotton. Injury on transgenic cotton did not exceed current economic thresholds of 15 to 25% injured flower buds, at any time in the season while injury on non-transgenic cotton exceeded the economic threshold frequently. The integrated crop ecosystem management model was used to estimate yields of non-transgenic and transgenic cotton at densities of 0, 0.1, 1, 5, 10, 25, and 50 small tobacco budworm larvae per meter of row from emergence to harvest. Density-yield response functions and response surfaces of insect infestations at two week intervals, were developed from these data and the number of degree-days from emergence. The response surface for transgenic cotton was flatter than the response surface for non-transgenic cotton showing much less susceptibility to tobacco budworm densities. Economic injury levels and economic thresholds will need to be raised for these insects when they attack transgenic cotton.