Some properties of a small RNA virus isolated from larvae of Heteronychus arator [Coleoptera: Scarabaeidae] are described. The virus develops in the cytoplasm of gut and fat body cells and is 30 nm in diamter. The sedimentation coefficient of virus was 137S and the buoyant density was 1-33 g/ml. The single stranded RNA was isolated as two species which sedimented at 22S and 15S, corresponding to mol. wt. of 1-0 and 0-5 X 10(6) respectively. The base ratios of the two RNA species were 22 S: A=29-0, C=24-1, G¿-8, U=24-1; 15S:A=26-4, C=23-8, G=24-7, U=25-2. There was one major polypeptide, mol. wt. 40 000 and two minor polypeptides in the virus. The virus was infective for Galleria mellonella and some other insects, but was not infective for mice. No evidence was obtained to suggest that the two RNA species were present in different particles.