The performance of various B.t. cotton varieties was evaluated against Helicoverpa zea (Boddie)(bollworm) in North Carolina in 1997 and 1998. The test was designed to determine if any inherent differences were present among the different cultivars with respect to their susceptibility to bollworm (i.e. differences in fruit damage and larval populations), their maturity profiles, and yields. A pyrethroid (Karate™) was also used to determine if treatment with this insecticide had any significant yield benefit in B.t. cotton and how yield relationships varied among varieties. Results from plant mapping data for 1997 and 1998 confirmed that ST 4740 BG was a significantly later maturing variety compared to the others tested. ST 4740 had significantly lower numbers of first and second position bolls and lower values of percent open bolls compared to the other varieties. Results also showed that non-pyrethroid treated B.t. cottons sustained considerable damage, and yield loss ranged from 6.6-31.7% compared to pyrethroid treated B.t. cottons. ST 4740 BG untreated subplots suffered significantly more damage and subsequently greater yield loss than all other untreated varieties.