Some field populations of bollworm, Helicoverpa zea (Boddie), in the southeastern U.S. have evolved resistance to Bacillus thuringiensis Berliner (Bt) toxins Cry1Ac and Cry2Ab produced by transgenic cotton, Gossypium hirsutum L. Decreased susceptibility to Cry1Ac, the toxin in first-generation Bt cotton (Bollgard), was detected in laboratory diet bioassays and was associated with increased survival on Bollgard plant tissues and control problems in the field. Bollgard cotton is no longer registered in the U.S. and has been replaced primarily by Bt cotton that produces Cry1Ac and Cry2Ab (Bollgard II). Laboratory diet bioassay results showed that the percentage of field populations resistant to Cry2Ab increased from 0% in 2002 to 50% in 2005. In bioassays from 2003 to 2006, 14 field-derived strains of bollworm had &lt;50% mortality at 150 µg Cry2Ab per milliliter of diet, the highest toxin concentration tested. As with the recently reported resistance of pink bollworm, Pectinophora gossypiella (Saunders), to Bollgard cotton in India, laboratory diet bioassays provide the critical data documenting genetically based decreases in susceptibility of bollworm field populations to Bt toxins Cry1Ac and Cry2Ab. Resistance to Bt toxins linked with increased damage to Bt crops in the field has now been documented in at least four target lepidopteran pests. Systematic analyses of these cases, as well as the many cases in which pests remain susceptible to Bt crops, can provide knowledge useful for increasing the durability of current and future generations of transgenic insecticidal crops.