Studies were conducted to determine if cotton bollworms (CBW) and beet armyworms (BAW) possess the genetic variation necessary to respond to selection for improved tolerance of the Bt toxin Cry1Ac. BAW individuals that were the fastest to reach pupation on Cry1Ac-treated diet (approx. the first 20% to pupate) produced offspring that developed signficantly faster on Cry1Ac diet than their parental-control strain. In addition, after four generations of mating, this group developed significantly faster on transgenic-Bt cotton leaves (cv. NuCOTN 33B) than the control strain. Full-sib families of CBW exhibited significant amounts of heritable variation for tolerance of Cry1Ac (54% of the total variation). Follow-up tests with these families further supported the heritable nature of Cry1Ac tolerance in CBW.