Cotton plants expressing the CrylAc toxin from Bt are near to commercial release. These plants have been developed to aid in the control of heIiothis caterpillars (both Helicoverpa armigera and H punctigera ), the primary targets of insecticide usage in cotton production. While it was hoped that these plants would give high enough expression of toxin to control heliothis throughout the growing season, it is now clear that some larvae can survive to pupation late in the season. The decline, however, is more of a problem for resistance management, because of the chance that at some stage during the growing season. the level of Bt toxin in the plants will be at a level that allows preferential survival of resistant, but not susceptible, individuals. We are examining the potential of transgenic Bt cotton plants to select for resistance in H. armigera. In particular, we need to understand what environmental factors influence the expression of the Bt toxin. At the beginning of our study in 1993, nothing was known about the cause of the decline in toxin levels, nor were techniques available that would enable us to quantify those levels. Thus, we have had to focus on developing the necessary techniques to study the performance of transgenic plants grown under field conditions.