The potential of Pandora neoaphidis (Remaudiére & Hennebert) Humber as a fungal pathogen for the control of the tobacco aphid, Myzus nicotianae Blackman, on tobacco was evaluated in a 4-year study between 1992 and 1995. The objectives of this study included determination of the seasonal incidence of P. neoaphidis in populations of tobacco aphid on tobacco and non solanaceous host plants, within plant distribution of the pathogen in aphids on tobacco, influence of tobacco cultivars and cultural practices on the incidence of the pathogen, methods of artificially introducing the pathogen into tobacco aphid populations and their potential in controlling aphids relative to chemical control, virulence of the Virginia isolate of the pathogen to tobacco aphid and green peach aphid, M. persicae (Sulzer), from different geographic locations in the eastern United States, and influence of temperature and type of substrate on the developmental morphology of the pathogen. Infections as high as 91% occurred in aphid populations on tobacco under favorable weather. The pathogen survived at moderate levels in the red morph of tobacco aphid on non solanaceous hosts during fall, parts of winter and spring. P. neoaphidis infections in aphids tended to increase towards the upper leaf positions. Incidence of the pathogen in aphid populations varied widely on various cultivars and types of tobacco. Planting date, topping of tobacco, and stage at which tobacco was topped did not influence the incidence of P. neoaphidis in tobacco aphids. Artificial introduction of P. neoaphidis successfully established infections in tobacco aphids, but failed to prevent the build up of aphid populations. P. neoaphidis was equally virulent to the red and green morphs of the tobacco aphid, and the green peach aphid. The developmental morphology of P. neoaphidis was influenced by temperature and was similar on the surfaces of living substrates, tobacco aphid and tobacco leaf, but different on the inert surface of the coverslip. / Ph. D.