Viruses are common in asexual Aspergilli but not in sexual Aspergilli. We found no viruses in 112 isolates of the sexual Aspergillus nidulans. We have investigated factors that could play a role in preventing the spread of mycoviruses through populations of A. nidulans. Experiments were performed with A. nidulans strains infected with viruses originating from A. niger. Horizontal virus transmission was restricted but not prevented by somatic incompatibility. Viruses were transmitted vertically via conidiospores but not via ascospores. Competition experiments revealed no effect of virus infection on host fitness. Outcrossing was found to limit the spread of viruses significantly more than selfing. It is concluded that the exclusion of viruses from sexual Aspergilli could be due to the formation of new somatic incompatibility groups by sexual recombination.