Transgenic cotton producing Bacillus thuringiensis (Bt) toxin Cry1Ac that kills pink bollworm, Pectinophora gossypiella, has accounted for more than half of Arizona’s cotton since 1997. Despite the wide use of Bt cotton and surprisingly high frequency of pink bollworm resistance to Cry1Ac detected in 1997, Bt cotton has remained extremely effective against pink bollworm. Laboratory bioassays show that the frequency of resistance to Cry1Ac did not increase from 1997 to 2003. Field-based estimates also show sustained efficacy during this period. In laboratory-selected strains with up to 3,100-fold resistance to Cry1Ac and survival on Bt cotton plants, resistance is linked with three recessive mutations in the gene encoding a cadherin protein that binds Cry1Ac. DNA-based monitoring of field-sampled individuals using the polymerase chain reaction (PCR) indicates that the frequency of the three identified resistance alleles remains low. Other examples of delayed resistance to Bt crops have been reported from Australia, China, and elsewhere in the USA. Factors delaying pest resistance to Bt crops include refuges of non-Bt crops, low initial resistance allele frequency, recessively inherited resistance, fitness costs, and incomplete resistance. The success of Bt crops exceeds expectations of many, but does not preclude future resistance problems.