Production of molecules with toxic activity by genetically transformed symbiotic bacteria of pest insects may serve as a powerful approach to biological control. The symbiont, Enterobacter gergoviae, isolated from the gut of the pink bollworm (PBW), has been transformed to express Cyt1A, a cytolytic protein toxin lethal to mosquito and black fly larvae, as a model system. These transgenic bacteria might be used to spread genes encoding insecticidal proteins to populations of agricultural insects or as replacement for chemical insecticides such as malathion used in bait formulation to control specific insect pests, because of extreme public pressure against organophosphate pesticide spraying.