The larvae of the spotted cutworm collected from grass fields at the beginning and the end of winter season were reared on an artificial diet in the laboratory. The larvae which died in the course of rearing were microscopically examined. In April and May, 1983, the main causative agents of diseased larvae were identified as nuclear polyhedrosis and granulosis. Two different types of polyhedra were observed: one was tetragonal and the other was trigonal in shape. Alkaline-liberated virions were rod-shaped and measured 80-190×320-380nm in the tetragonal type and 95-165×345-385nm in the trigonal type. The occlusion bodies of GV were ellipsoidal and measured 0.2-0.3×0.4-0.6μm. Nucleocapsids of GV were slightly curved and measured 70-72×400-405nm.