Development, survival, and fecundity of beet armyworm (BAW) were determined on Bt and non-Bt cotton. The following treatments were established: 1) conventional-conventional, 2) conventional-transgenic, 3) transgenic-conventional, and 4) transgenic-transgenic, which resulted from the combination of two collection sources of BAW larvae (conventional and transgenic cotton) and two diets for feeding their offspring in the laboratory (conventional variety DP 5690 and transgenic variety NuCOTN 35B). BAW growth, survival and fecundity were negatively affected by the Bt toxin contained in transgenic cotton. Larval weight and size, as well as pupal weight, were reduced by the toxin. Developmental times were similar on Bt and non-Bt cotton. Larvae survival decreased from 72% in insects not exposed to the Bt toxin to 26% in those fed on cotton leaves with or without the toxin, whose parents were exposed to it. Only when parents and their offspring were exposed to the toxin, a reduction of fecundity occurred.