The growth rate and pathogenicity of strains LeekBc-10 and GarlicBc-2 of Botrytis squamosa have been determined in this study. Double stranded RNAs (dsRNAs) in strain LeekBc-10 were cDNA cloned and the resulting cDNA sequences were used to infer proteins and taxonomic status. The results showed that the strain LeekBc-10 was inferior in growth on potato dextrose agar and more vulnerable to infection of garlic leaves, compared to the strain GarlicBc-2. Two dsRNAs, dsRNA-1 and dsRNA-2, were detected in the mycelia of strain LeekBc-10. Sequences of dsRNA-1 and dsRNA-2 are 91.7% and 97.7% identical to the corresponding sequences of dsRNA-1 and dsRNA-2 from Botrytis porri RNA virus 1 (BpRV1). Therefore, the dsRNAs in the strain LeekBc-10 are the genome of a RNA virus, designated as Botrytis squamosa RNA virus 1 (BsRV1). Two open reading frames (ORF), ORF I and ORF II, were predicted on the dsRNA-1 and dsRNA-2 of BsRV1, respectively. They putatively encode two proteins with 1 901 and 1 788 amino acid residues respectively, which were 97.1% and 98.7% similar to the proteins putatively encoded by ORF I and ORF II of BpRV1 respectively. The protein encoded by ORF I of BsRV1 contains the RdRp_4 (RNA-dependent NA polymerase) superfamily domain. The RdRp of BsRV1 has eight conserved motifs for viruses belonging to Totiviridae, Chrysoviridae and Megabirnaviridae and is 19%-23% identical to those encoded by the viruses in these families. This study suggests that BsRV1 and BpRV1 belong to the same virus species, possibly representing a novel virus family.