Laboratory bioassays were carried out to determine susceptibility levels of the pink bollworm (PBW) to the Bt endotoxin CryIAc. PBW larvae survival was also evaluated in Bt and non-Bt cotton bolls, under field conditions. Survival of PBW larvae from the different generations was reduced as the toxin concentration increased. PBW larvae did not survive beyond the first instar at a concentration of 1.0 mg of CryIAc/ml of diet in laboratory bioassays. Weight and size of PBW larvae from the fourth generation were reduced with increasing concentrations of the endotoxin CryIAc. There were no large PBW larvae (third and fourth instars) in naturally or artificially infested Bollgard^®; cotton bolls. PBW larvae did not develop beyond second instar in Bollgard^®; cotton bolls. Therefore, current PBW population from the Comarca Lagunera is highly susceptible to the endotoxin CryIAc.