Marketing of cotton and corn expressing Bt toxins in the Cotton Belt is a goal of seed companies. Cotton bollworm infests both crops during several generations and both crop species may express similar types of Bt toxins. Resistance evolution in bollworm to CryIAc toxin in cotton is expected to be affected by the culture of CryIAb corn, and visa versa. A wide area stochastic computer simulation model was developed to examine resistance evolution relationships in a cropping system containing both Bt cotton and Bt corn. This paper presents parameters selected for use in the model and discusses aspects of bollworm resistance evolution to Bt toxins.