One approach in the drive by the cotton industry to introduce more effective and more sustainable methods of bollworm control than are presently available, has been to develop a new generation of insecticides based on naturally occurring insect pathogens such as the nuclear polyhedrosis viruses (NPVs). From an environmental standpoint, these bio control agents have the advantage of a much narrower target range, leave no toxic residues, and have no adverse effects on human or animal health. Although NPVs have been applied as insecticides since the 1940's, the relatively long time it takes to kill the insect target (often several days) has generally limited their use to low value, mostly perennial cropping systems eg forest plantations. This is no longer the case. With the advent of genetic engineering technologies in the late 1980's, has come the opportunity to significantly enhance their speed of kill&quote;'. As a consequence both the type of insect pests which can be controlled and the cropping systems which can be targeted have now broadened. In 1988, CSIRO's Division of Entomology initiated a programme to genetically modify an Australian NPV isolated, from the cotton bollworm, Helicoverpa armigera (HaNPV)for increased speed of kill (see Christian & Richards in these proceedings).