Resistance to pyrethroid insecticides was first documented in Helicoverpa zea (Boddie) in 1990. Since then the number of reported cases of resistance has increased in number and distribution, posing a threat to H. zea control. In 2004 a few commercial growers in Indiana reported control failures. In 2005 control of H. zea by pyrethroids in small plot trials was low. Two populations of H. zea collected from Indiana, and one population from Illinois were compared with a susceptible laboratory population for resistance to bifenthrin using topical bioassays on third instar larvae. Topical bioassays were also conducted with larvae pre-treated with the synergist piperonyl butoxide to determine if the nature of resistance is metabolic or knock-down resistance. Data from the bioassays were used to calculate LD50s and resistance ratios for all three populations, with and without piperonyl butoxide.