Transmission of dsRNA viruses between homo- and heterokaryotic mycelia paired on agar plates and into conidia has been studied in Heterobasidion annosum. Horizontal transmission of dsRNA occurred between both homo- and heterokaryotic isolates, as well as between isolates belonging to different intersterility groups. The proportions of vertical transmission into conidia were 3% and 55%, respectively, for the two isolates included in the study. RT-PCR of dsRNA and PCR-RFLP of mitochondrial markers were used to confirm transmission of dsRNA between the cytoplasms of different mycelia. The identity of nuclei and nuclear migration during experiments were verified using PCR-RFLP of several nuclear markers.