Neuronal sensitivity to the pyrethroid insecticide, allethrin was measured in larvae of the bollworm, Helicoverpa zea (Boddie). An established neurophysiological assay was used to record and compare sensitivities in laboratory-susceptible and field strain larvae collected from a wild host, corn, conventional and transgenic (NuCOTN33) cotton in Louisiana during 1995 and 1996. Laboratory and field-collected larvae were equally target sensitive during 1995, but a low frequency of insensitive individuals was detected in conventional and transgenic cotton collections during 1996.