Mycelium of Pleurotus ostreatus var. florida with a decreased growth rate contained seven double-stranded RNA segments and isometrical virus particles with diameters of 24 and 30 nm. Mycelium with a normal growth rate lacked dsRNA. Protoclones from virus-containing mycelium contained one to seven of these dsRNA segments in varying concentrations. The exact correlation between slow growth and the presence of dsRNA molecules could not be established. Infection of virus-free protoplasts with PEG-precipitated virus particles resulted in mycelium that stably maintained the 2.4 kbp dsRNA.