In an examination of a laboratory colony of Helicoverpa zea (Boddie), we found heritable variation to the Bt toxin, CryIAc. Based on this preliminary finding, we used a similar bioassay to monitor field populations of H. zea and Heliothis virescens F. There was significant variation in the log weights (mg) (measure of tolerance to CryIAc both within and between field colonies for both species). The average log weights (mg) and % mortality of colonies were negatively correlated in both species. Additional testing of one colony of H. virescens found that susceptibility to CryIAc could be transmitted to offspring. We are currently conducting follow up tests on the remaining field colonies to determine if the variation in tolerances has a heritable basis.