Technical Abstract: Cottons containing the gene that mediates production of crystalline insect toxic protein (Bt cottons) have been grown commercially in Arizona since 1996. Extensive PBW population sampling has demonstrated continuing susceptibility to Bt with averages of less than one percent seasonal boll infestations reported. In contrast, under laboratory conditions, tolerance to increasing doses of lyophilized powder made from transgenic cotton leaves increased after three generations of selection. Additionally, selections of the resistant strain by exposure to the Bt toxic protein incorporated in artificial larval diet have produced strains 300 to 3100-fold more tolerant than laboratory culture non-resistant reference strains. The difference between field and laboratory results remains unexplained.