Abstract:A series of experiments with a newly finding nuclear polyhedrosis virus of the oak caterpillar (Cyclophragma undans Walker) have been carried out. This virus, causes the larvae of the oak caterpillar lose their appetite and become slow in movement and yellowish brown in color after infection. The inne tissues of the dead larvae were fluidized and flowed out from the damaged skins which were fragile. The laboratory virulence tests showed that the mortality of the 6—8th instar larvae come up to 94.7% trated with 0.4×108 PIB/ml CuNPV.Under electronic microscope the polyhedral bodies of CuNPV are tetrangonal, pentagonal or irregularly polygonal, and are measured to be 1.8—2.9μm, sometimes up to 4.0μm in size. Their protein lattice is of a pattern of regular dots and rows. The distance of two neighouring grides of the lattice is 4.40nm. The dots of the protein lattice are about 2.5nm in diameter. The virus bundle is enveloped by lax protein capsule with a thickness of 15—20nm. The polyhedroses are MEV, most of them have 2—4 virions which are rod-shaped and are measured 250—300×30—40nm.The viral DNA released from virions of CuNPV are found to be the circular or open circular molecules with countour length of 36.1 μm on the average. The molecular weight of the viral DNA is 71.12×106 daltons.This virus as an insecticide is safe to vertebrates by experiments.