Field studies were undertaken at 5 sites over 2 years to determine the efficacy of 2 lines (Mon-81 in 1992 and Mon-531 in 1993) of transgenic B.t. cotton, primarily against economically important lepidopterous cotton pests. These studies relied on natural infestations of the cotton pest complex in North Carolina. Particular attention was focused on damage caused by the cotton bollworm, Helicoverpa zea (Boddie), and the tobacco budworm, Heliothis virescens (F.). Overall, excellent control of the bollworm complex and the European corn borer, Ostrinia nubilalis (H�bner), was observed. However, 14% boll damage from H. zea was recorded in the B.t.-untreated plots at the Edgecombe-93 site. At the Onslow-92 and Onslow-93 sites damage from another economically important pest, the green stinkbug, Acrosternum hilare (Say), was inflicted on some plots in amounts high enough to quantify.