Statewide monitoring of pink bollworm susceptibility to Bt toxin was conducted in Arizona in 1997 and 1998. In bioassays in which Bt toxin was incorporated into insect diet, a concentration of 10 µg/ml Cry1Ac caused >80% mortality of all field populations. LC50s of Arizona populations ranged from 0.35 to 1.7µg Cry1Ac/ml of insect diet. A laboratory strain of pink bollworm had an LC50 of 0.53 µg/ml. A selection experiment was conducted in which survivors of bioassays of 3.2 and 10 µg/ml of Cry1Ac from all Arizona populations tested in 1997 were pooled and reared for one generation on diet containing 10 µg Cry1Ac/ml. Selection produced a strain of pink bollworm that was 100- to 460-fold less susceptible to Cry1Ac, than were the field populations from which it was derived. Preliminary results from greenhouse evaluations concerning survival of this resistant strain on Bt cotton showed that larvae were able to complete development in bolls of Bt cotton, pupate, and successfully reproduce. We conclude from these preliminary results that the type of resistance we have isolated in the laboratory is likely to negatively impact field performance of Bt cotton in Arizona in the future. However, we found no evidence that field performance of Bt cotton has yet been compromised. In large field trials, in-field refuges of non-Bt cotton were contrasted with the standard, external refuges. During both 1997 and 1998, in-field refuge plots yielded comparable to, or better than, plots of 100% Bt cotton, yet they produced substantial numbers of pink bollworm late in the season on non-Bt plants.