Increased tolerance to Cry1Ac protein was found in a population of fall armyworms, Spodoptera frugiperda (J. E. Smith), after selection for a single generation with transgenic Bacillus thuringiensis Berliner (Bt) cotton foliage. When fed Cry1Ac treated artificial diet, larvae whose parents had fed on transgenic Bt cotton leaves had significantly higher larval weights and a shorter time to pupation than those larvae whose parents had fed on conventional cotton leaves. In addition, there was no evidence to suggest any fitness or vigor differences existed from progeny of fall armyworms that fed previously on conventional or transgenic Bt cotton. Furthermore, tolerance of fall armyworms to Cry1Ac had a heritable component in the subsequent generation based on larval weights and time to pupation. These data show that using a common approach designed to control all intrinsically tolerant lepidopteran species of transgenic Bt cotton identically may not be desirable.