An insect virus, called Manawatu virus (MwV), was isolated from a larva of the New Zealand grass grub Costelytra zealandica (White) (Coleoptera: Scarabaeidae). MwV was serologically related, but not identical, to several insect nodaviruses. The single capsid protein of MwV was 40,000 MW, the same as black beetle virus (BBV), but virus particles had a different electrophoretic mobility from BBV. The bipartite RNA genome, like other nodaviruses, consisted of two species of MW 1.1 and 0.46 million. MwV particles sedimented at 142 S and had an estimated density in neutral CsCl of 1.366 g/ml compared with 1.352 g/ml for BBV. The serological and physico-chemical properties, compared with other nodaviruses, indicate that MwV is unique.