Tobacco budworm (TBW) populations from different cotton producing areas of northwestern Mexico were monitored in 1998 and 1999 for response to the Cry IA (c) toxin, which is expressed in BOLLGARD^® cotton. Bioassays were carried out on field-collected populations of TBW and compared to a laboratory susceptible population. Overlay concentrations of 0.05 m     g/ml of the toxin were applied on lepidoptera diet and data on mortality, larvae reaching 3^rd instar and percent growth inhibition were obtained 5 days after treatment. Results indicated that this dosage prevented larvae from reaching the 3^rd instar, indicating the susceptibility to the toxin in the populations evaluated. The comparison between weigh of treated versus untreated larvae produced a percentage of growth inhibition, and this ranged from 96% to 98% in 1998, and it was from 98% to 99% in 1999. These data indicate the high susceptibility of the CryIA(c) toxin to the treated larvae. Monitoring of resistance has not indicated any shift to resistance to this toxin in populations from northwestern Mexico.