Abstract: A cytoplasmic polyhedrosis virus causing an epizootic disease was isolated from the body of Parasa sinica Moore in the region of Nanjing in 1983.This virus infringed midgut of larvae, forming polyhedra in the cytoplasm of epithelial cells. The infected larvae showed reduced body size and lost appetite. The excreted feces became milky white in later stage. The midgut became milky white also, when a piece of midgut was taken and observed under microscope, many polyhedra were found.Under the electron microscope, the polyhedron is tetrahedral and hexahedral or of irregular shape, but the virion is nearly spherical with tubular structure. The LC50 inclusion body CPV concentration for the 3rd-4th instar Parasa sinica larvae was 1.21 x 10~5PIB/ml. The experiments showed that the mortality rate depended on the concentration of the virus.According to the international classification system the virus is considered to belong to Cypovirus in family Reoviridae. It is one of the species of cytoplasmic polyhedrosis viruses (CPV) .