Twelve double-stranded (ds) RNA segments were detected from a hypovirulent strain W370 of the white root rot fungus Rosellinia necatrix. The estimated molecular weights ranged from 0.41 x 10(6) to 2.95 x 10(6). Full length cDNA clones for eight segments were obtained. Northern blot analysis suggested that each segment was genetically unique. The nucleotide sequences of eight full length dsRNA segments were determined. One long open reading frame was found in each segment. Conserved sequences at the 5'-end (5'-ACAAUUU-3') and at the 3'-end (5'-UGCAGAC-3') were identified in all eight segments. Segment-specific panhandle structures, formed by inverted terminal repeats, were also found in all segments. Comparative analyses of the predicted translational products of eight dsRNA segments showed that the deduced amino acid sequence partially matched those of the Reoviridae family members: Colorado tick fever virus, Nilaparvata lugens reovirus, and rice black streaked dwarf virus. The results suggested that W370 dsRNA is derived from a new member of the family Reoviridae detected in fungus.