The performance of four cotton genotypes was evaluated against Helicoverpa zea (Boddie) in field and greenhouse studies in North Carolina in 1999. The field studies were designed to determine if two Bollgard II™ lines exhibited an increased efficacy against bollworm compared to the current commercial Bollgard™ variety. The impact on yield of supplemental bollworm control with pyrethroids was evaluated. Two greenhouse studies were designed to compare the Bollgard II lines with the commercial Bollgard variety with respect to efficacy against a field strain of bollworm only in one study and against a field strain and a strain which had been selected in the laboratory for thirteen generations for tolerance to the Cry1A(c) toxin in the second study. Results from the field studies indicated that Bollgard II lines (15813 and 15985) sustained significantly lower terminal, square, and boll damage compared to the commercial Bollgard variety (DP50B) and a conventional cotton variety (DP50). The yield response of Bollgard genotypes to pyrethroid oversprays varied among test sites as expected under modest bollworm population levels. In greenhouse studies, Bollgard II lines significantly reduced penetration of fruiting structures for the field and laboratory-selected strains of bollworm compared to the Bollgard variety. However, larval survival and fruit damage by the laboratory-selected bollworm strain were higher than that of the field-collected, non-selected strain.