Double-stranded RNA (dsRNA) viruses and single-stranded RNA(ssRNA) viruses were detected in a strain of Pleurotus mushroom cultivated in a farm. Those fungal virsus were purified in the pH 6.0 or pH 7.2 using CsCI or Cs$_{2}$SO$_{4}$ buoyant density centrifugation. Each viral particles were not completely separated at any trials. However, mushroom bacili-form virus contains a single major nucleic acid with 0.7 Kb ssRNA, which might code for 20 Kd viral capsid protein. The dsRNAs are encapsidatred into spherical-form viruses, whereas ssRNA viral genomes are encapsidated into two different sizes of bacili-form particles. A healthy-looking mushroom also contained some spherical-form viruses with dsRNAs. Laboratory strains of Pleurotus ostreatus and a cultivated strain of P. sajor-caju did not show any viral particles. Mushrooms with specific disease symptoms. however, contained at least four different sizes of spherical-form viruses. Thus, we concluded that a bacilli-form virus case a severe disease symptoms of adnormal on mushroom development.