Viruses typically encode the capsid that encases their genome, while satellite viruses do not encode a replicase and depend on a helper virus for their replication(1). Here, we report interplay between two RNA viruses, yado-nushi virus 1 (YnV1) and yado-kari virus 1 (YkV1), in a phytopathogenic fungus, Rosellinia necatrix(2). YkV1 has a close phylogenetic affinity to positive-sense, single-stranded (+)ssRNA viruses such as animal caliciviruses(3), while YnV1 has an undivided double-stranded (ds) RNA genome with a resemblance to fungal totiviruses(4). Virion transfection and infectious full-length cDNA transformation has shown that YkV1 depends on YnV1 for viability, although it probably encodes functional RNA-dependent RNA polymerase (RdRp). Immunological and molecular analyses have revealed trans-encapsidation of not only YkV1 RNA but also RdRp by the capsid protein of the other virus (YnV1), and enhancement of YnV1 accumulation by YkV1. This study demonstrates interplay in which the capsidless (+)ssRNA virus (YkV1), hijacks the capsid protein of the dsRNA virus (YnV1), and replicates as if it were a dsRNA virus.