The increasing difficulty of controlling cotton bollworm, Helicoverpa armigera, in Australia due to its resistance to many chemical insecticides and the pressure to reduce the usage of chemicals led to the adoption of transgenic cotton as the key component of its pest control strategy by the Australian cotton industry. The commercial varieties available to date are based on the INGARD@ technology and contain an insecticidal protein (Cry1Ac) which was obtained from the bacterium Bacillus thuringiensis (Bt). As INGARD@ varieties contain only one insecticidal protein, growers and researchers were concerned that cotton bollworm might become resistant to the transgenic cotton. At the inception of the project, resistance to the insecticidal proteins had not yet been demonstrated for any cotton pests. However, we were aware that the diamondback moth had become highly resistant to Bt sprays in many parts of the world. Subsequently, some 26 species of insect pests, including Heliothis virescens, have shown the ability to develop substantial resistance to Bt proteins. The naturally high tolerance of H. armigera for the Cry1Ac protein produced by INGARD and the decline in insecticidal activity through the latter part of the season (Fitt, 1998) suggest that inappropriate usage could well lead to INGARD@ losing its ability to prevent H. armigera damage. Consequently, the CRDC and CSIRO funded a project to investigate the potential for cotton bollworm to develop resistance to transgenic cotton and evaluation of the consequences of such resistance.