Transgenic cotton,Gossypium hirsutum L., expressing thecry1Ac andcry2Ab genes fromBacillus thuringiensis (Bt) Berliner varietykurstaki in a pyramid (Bollgard II) was widely planted for the first time in Australia during the 2004–2005 growing season. Before the first commercial Bollgard II crops, limited amounts of cotton expressing only the cry1Ac gene (Ingard) was grown for seven seasons. No field failures due to resistance to Cry1Ac toxin were observed during that period and a monitoring program indicated that the frequency of genes conferring high level resistance to the Cry1Ac toxin were rare in the major pest of cotton,Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Before the deployment of Bollgard II, an allele conferring resistance to Cry2Ab toxin was detected in field-collectedH. armigera. We established a colony (designated SP15) consisting of homozygous resistant individuals and examined their characteristics through comparison with individuals from a Bt-susceptible laboratory colony (GR). Through specific crosses and bioassays, we established that the resistance present in SP15 was due to a single autosomal gene. The resistance was recessive. Homozygotes were highly resistant to Cry2Ab toxin, so much so, that we were unable to induce significant mortality at the maximum concentration of toxin available. Homozygotes also were unaffected when fed leaves of a cotton variety expressing the cry2Ab gene. Although cross-resistant to Cry2Aa toxin, SP15 was susceptible to Cry1Ac and to the Bt product DiPel.