Field populations of Helicoverpa zea (Boddie) and Heliothis virescens F. from the eastern one-half of the United States cotton belt were monitored from 1996 to 1998 for tolerance to the Bacillus thuringiensis toxin Cry1Ac. Spray chamber analyses of H. zea from the Mississippi Delta revealed a slight decrease in susceptibility to Cry1Ac during the 3-year period. Areas producing the greatest change in tolerance to Cry1Ac had a greater percentage of their acreage planted in Bt cotton. In general, tolerances of H. virescens populations did not change, with the single exception being the third generation of H. virescens collected from the Mississippi Delta. The small changes in tolerance in H. zea reported herein suggested that even though populations were slightly more tolerant of Cry1Ac, they were not at a level to cause control failures in the field.