Knowledge of mycoviruses is so new that most scientists are barely aware that they exist, yet they are widespread and affect many parameters. Their possible effect on the levels of toxins and metabolites produced by fungi enhance their significance in environmental health research. Evidence for the occurrence of a lysogenic virus in yeast was presented as early as 1936 (1), and the viruses of higher fungi were first suspected in 1950 when Sinden and Hauser (2) reported a degenerative disease of mushrooms. Olpidium brassicae was shown in 1958 to be the vector of the virus which causes the big vein disease of lettuce but the evidence did not suggest that the fungus supported multiplication of the virus (3, 4). The first virus of a fast growing fungus was reported by Ellis and Kleinschmidt (5, 6). The discovery resulted from several years of intensive investigation of the active factor in Penicillium stoloniferum responsible for the stimulation of interferon in test animals. Lampson et al. (7), working with another antiviral substance from P. funiculosum also capable of inducing interferon (8), found this activity associated with a doublestranded RNA extracted from mycelia of this mold. Subsequently, the RNA extracted from purified P. stoloniferum mycovirus was shown to be double stranded in nature (9).