Virus-like particles (VLPs, named HmTV1-17), about 40 nm in diameter were found in the violet root rot fungus Helicobasidium mompa Tanaka strain No. 17, which had been isolated from an apple tree. Purified preparations of HmTV1-17 contained two species of double-stranded RNA (dsRNA), designated 17L and 17S. cDNAs were constructed from HmTV1-17 genomic dsRNAs purified using CF-11 cellulose column chromatography. The sequences of 17L and 17S cDNA comprised 5,207 and 2,096 bp, respectively. Although 17S has no large open reading flame (ORF) on either strand, 17L has two large overlapping ORFs. The 5' located ORF1 encodes the coat protein (CP, 788 amino acids), whereas the gene product of ORF2, which is in the -1 frame relative to ORF1, shows the typical features of a RNA dependent RNA polymerase (RDRP, 845 amino acids). Phylogenetic analysis based on RDRP showed that HmTV1-17 is closely related to Sphaeropsis sapinea SsRV1, a member of the genus Totivirus from filamentous fungus S. sapinea.