Raspberry bushy dwarf idaeovirus (RBDV) has isometric, 33 nm diam. particles and a bipartite RNA genome. Sequencing of the larger component (RNA-1) showed that it consists of 5449 nucleotides and contains 1 large open reading frame encoding a putative translation product with a calculated Mr of 190 000. Comparisons of this polypeptide with non-structural proteins of other plant viruses revealed significant homologies with those of alfalfa mosaic alfamovirus (AlMV), brome mosaic bromovirus (BMV), cucumber mosaic cucumovirus (CMV) and tobacco mosaic tobamovirus (TMV). Thus RBDV belongs to the supergroup of 'Sindbis-like' plant viruses. The translation product of RBDV RNA-1 contains motifs characteristic of proteins with polymerase, methyltransferase and helicase activities, suggesting that this protein is involved in the replication of the viral RNA. Thus in RBDV, as in TMV, all 3 functional domains are combined in the single protein, whereas in AlMV, BMV and CMV these domains are distributed over the proteins encoded by RNA-1 and RNA-2. These findings support the idea that RBDV should be placed in a distinct virus genus for which the name idaeovirus has been proposed.