Interest in insect baculoviruses has been stimulated by their potential as biological control agents and intensive research has focused on the improvement of their pesticidal properties by genetic engineering. Much of this effort has been directed at Autographa californica nucleopolyhedrovirus (AcMNPV). One approach taken has been the deletion of the AcMNPV egt gene. The egt gene product, ecdysteroid UDP-glucosyltransferase (EGT), catalyses the conjugation of insect ecdysteroid hormones with UDP-sugars (O'Reilly et al., I99I,