Intra- and interspecies transfer of dsRNA viruses between black Aspergilli and Aspergillus nidulans strains has been investigated using protoplast fusion. We found interspecies transfer of virus in all combinations of black Aspergillus and A. nidulans strains and vice versa. Using the same conditions, intraspecies virus transfer among heterokaryon incompatible strains was also tested. Whereas such transfer was always found among A. nidulans strains, transfer among black Aspergilli was frequently unsuccessful. The lack of virus transfer between black Aspergillus isolates was further investigated by using a mitochondrial oligomycin resistance marker as a positive control for cytoplasmic exchange. These experiments showed independent transfer of the oligomycin resistance and dsRNA viruses during protoplast fusion of heterokaryon incompatible black Aspergilli. The inefficient transfer of dsRNA viruses between black Aspergilli is not caused by absolute resistance to viruses but may be related to heterokaryon incompatibility reactions that operate intraspecifically. Consequences for the dynamics of mycoviruses in populations of black Aspergilli are discussed.