Transgenic cotton varieties widely differ in the amount of expressed Bt throughout the growing season. These data suggest that the conventional parental background, for which the cry1Ac gene is inserted, is a factor contributing to expression differences. Although other environmental factors (e.g. irrigation and plant stress) could influence expression, soil composition did not contribute to Bt expression differences among varieties. These expression data will hopefully contribute to a variety-development program that incorporates high-yielding varieties with the best transgenic insect control.