Viruses of fungi (mycoviruses) were first discovered in diseased mushrooms. However the finding that the antiviral and interferon-inducing activities of extracts of apparently healthy isolates of a number of Penicillium species were due to the presence of double-stranded (ds) RNA arising from mycovirus infections sparked off an explosion of interest in what has now become a distinct area of virology. Two families of dsRNA mycoviruses are now established: the Totiviridae and the Partitiviridae which comprise isometric viruses with genomes of one or two dsRNA segments respectively. Virus isolates in both families often contain additional satellite dsRNAs, which may in some fungi (e.g. Saccharomyces cerevisiae, Ustilago maydis) code for "killer" proteins which are toxic to other sensitive strains of the same or closely related species. In Endothia parasitica, which causes chestnut blight disease, dsRNA is associated with hypovirulence and is enclosed in lipid-rich vesicles. In Ophiostoma (Ceratocystis) ulmi, which causes Dutch elm disease, dsRNA is associated with the mitochondria and, in some diseased isolates of the fungus, specific dsRNA segments are associated with reduction of cytochrome oxidase and respiratory deficiency, resulting in slow growth and abnormal morphology.