Transgenic cottons (Bt), Gossypium hirsutum L., have been grown commercially in Arizona since 1996. Bt transformed cottons contain a gene or genes from Bacillus thuringiensis Kurstaki (Berliner) that regulate production of insect toxic proteins. Outstanding pink bollworm (PBW), Pectinophora gossypiella (Saunders), population suppression has been achieved in transgenic cotton production systems through the 2005 growing season However, the threat of resistance development to the toxic proteins has been of continuing concern. There is an increasing need for information on pest interactions in transgenic cotton cultures to provide information to supplement resistant management programs and provide knowledge to prevent or delay reduced efficacy of the Bt system. We conducted laboratory studies from 2001 to 2005 to determine the effect of sublethal PBW feeding periods on larval development, mortality, pupation, and tolerance to the Cry1Ac toxic protein in 'NuCOTN 33B®' green cotton bolls, pollen and artificial diet. Higher larval mortality occurred and more days were required for pupal development following four- or seven-day feeding periods on 'NuCOTN 33B®' bolls compared with similar feeding periods on Delta and Pineland (DPL) 5415 non-toxin containing bolls. Mortality and larval development time decreased as the number of sublethal larval feeding generations increased suggesting adaptation to the toxic protein in bolls. Small numbers of larvae of a Bollgard®(BG®) PBW strain developed by feeding for 35 of 40 generations for four days in each generation on 'NuCOTN 33B®' bolls survived on 'NuCOTN 33B®' pollen and artificial diet containing Cry1Ac toxic protein, but not on cotton bolls. No survival of larvae of a susceptible DPL PBW control strain occurred when feeding on 'NuCOTN 33B®' pollen or bolls, or at high (10 µg/g) concentrations of Cry1Ac protein in artificial diet.