We have characterized members of the Mapputta group of 'bunyavirus-like' viruses in terms of morphology, structure, ultrastructural development and virus-directed RNA and protein synthesis. Our primary study has been with Maprik virus (MPK) as a representative of the group. The MPK virion is uniformly spherical (congruent to 90 nm diameter) and possesses a membrane envelope. Virus maturation is by budding into small vesicles in the perinuclear region. During infection of BHK cells which is cytopathic, maximum viral RNA synthesis is congruent to 20% of RNA synthesis in uninfected cells without actinomycin D. In Aedes albopictus cells, where high initial titres are followed by a persistent infection, maximum viral RNA synthesis is only 1.5% of levels in control cells. MPK virions contain three RNA species. MPK generates four virus-specific RNAs (L, M, S1 and S2) and three nucleocapsid species in infected BHK cells. Six virus-specific polypeptides are present in infected cells; the four largest correspond to L (a probable transcriptase component), G1 and G2 (envelope proteins) and N (the nucleocapsid protein), respectively. These results and complementary studies with Trubanaman and Gan Gan viruses indicate that the members of the Mapputta serogroup can be placed in the bunyavirus genus of the family Bunyaviridae.