Transgenic varieties of field corn that express the Cry1Ab B. thuringiensis (Bt) toxin in ear tissue present the potential of reducing ear feeding by the corn earworm, Helicoverpa zea (Lepidoptera: Noctuidae), and for reducing the size of populations of the insect infesting other host crops. Life history parameters of H. zea feeding on ears of conventional and Bt field corn varieties were measured in field plots in eastern North Carolina in 1997 and 1998. Transformation events investigated were Mon-810 and Bt-11. Bt corn was found to cause a steady mortality of larvae during development, but permitted 15-40% survival to the prepupal stage compared with non-Bt corn. Mortality of prepupae and pupae from Bt corn was also higher than from non-Bt corn, reducing overall adult production by 65-95%. The larvae that did survive grew more slowly on Bt than on non-Bt corn, and produced pupae that weighed 33% less. Pupation and adult eclosion were delayed by 6-10 d by feeding on Bt corn ears. Corn varieties expressing Bt in ear tissue have the potential to reduce H. zea ear feeding by up to 80%, and the potential to reduce populations emerging from ear-stage corn fields to infest cotton, soybean and other crops by around 75%. To have a measurable effect on area-wide populations, Bt corn varieties would need to be planted in large proportions of corn fields. Extensive planting of varieties such as those tested here, having only moderate effects on H. zea, would raise concerns about rapid evolution of resistance.