The gypsy moth, Lymantria dispar L., is one of the most damaging pests of the deciduous forests in the United States. It was accidentally introduced from Europe in 1868 by an amateur naturalist in eastern Massachusetts. High density gypsy moth populations are regulated primarily by a nuclear polyhedrosis virus (LdNPV). LdNPV is transmitted by feeding the LdNPV contaminated foliage or the contaminated egg chorion on the way out from the egg by a larva. In 1989, an entomophthoralean fungus, Entomophaga maimaiga Humber, Shimazu et Soper was discovered in the northeastern United States, which caused massive epizootic in both low and high density gypsy moth populations. My study focused on the interactions between E. maimaiga and LdNPV. Laboratory bioassays in which I inoculated gypsy moth larvae with LdNPV and E. maimaiga at the same time indicated that the majority of dually inoculated larvae die from E. maimaiga because of the shorter incubation period of E. maimaiga (5-7 days) compared to LdNPV (14 days) at 20$\sp\circ$C. When the larvae were inoculated with E. maimaiga, 10 days after LdNPV inoculation, there was an apparent synergistic effect of E. maimaiga with LdNPV. Dually inoculated larvae died producing LdNPV propagules, 1-2 days earlier than the larvae inoculated with LdNPV alone. Small-scale field experiments conducted in mesh-bags showed that artificial rainfall increases the E. maimaiga transmission. In a naturally occurring, moderate density gypsy moth population, I found that the LdNPV infection level was little affected by the presence of E. maimaiga. Host heterogeneity is suspected as one of the factors leading non-linear LdNPV transmissions. I showed that the host heterogeneity cannot explain the E. maimaiga epizootic observed in low density populations. I experimentally demonstrated this by comparing the E. maimaiga infection rates in feral (experienced the E. maimaiga/LdNPV epizootic in their parental generations) and laboratory reared (with no epizootic experience) larvae. This is probably due to the short period to which the North American gypsy moths have been exposed to E. maimaiga, so these gypsy moths have not had chance to develop resistance against E. maimaiga.