A quick, noninvasive technique for monitoring the presence and expression of transgenes in crops could benefit farmers and lessen the risk of transgene escape. On page 1125, Stewart and colleagues show that GFP is an accurate marker of expression of the Bt toxin gene, providing a low-tech way to detect transgenic plants in the field. They determined the best GFP variant for this purpose, and established that its expression had no apparent fitness costs to plants growing in the field.