NuCOTN 33b (Bt cotton) (Monsanto Company, St Louis, MO) carrying the gene for the Bacillus thuringiensis var. kurstaki (Berliner) insect toxic protein is a new advance in technology for pink bollworm (PBW), Pectinophora gossypiella (Saunders), control. To investigate grower concern for reduced efficacy in late-season because of breakdown or non-expression of the toxic protein, we compared the susceptibility of Bt and Deltapine 5415 (Monsanto Company, St Louis, MO) (non-Bt) cotton bolls to PBW at periodic intervals during the first and second cotton fruiting cycles. We placed >200 PBW eggs per boll on the inside surface of a bract of susceptible immature cotton bolls. The artificially infested bolls were later harvested and examined for evidence of PBW infestation. High percentages of both Bt and non-Bt cotton bolls had numerous larval entrance holes in the carpel walls of the bolls. Less than 1% of the Bt cotton bolls and over 70% of the non-Bt cotton bolls were found with living PBW larvae. Bt cotton bolls of the late-season second fruiting cycle were as resistant to PBW infestation as Bt cotton bolls of the first fruiting cycle.