In general, the virulence of Rhizoctonia Solani Kühn, shown on potato stems, decreased as the proportion of inoculum, which was grown in steam sterilized soil, increased. Isolates highly pathogenic in a 1:15 soil inoculum—natural soil mixture were relatively weak in pure soil inoculum protected from outside contamination.Sclerotia developed and mycelia massed much more frequently and abundantly in the higher concentrations of soil inoculum than in the 1:15 soil-inoculum–natural-soil mixture. Thus, the evidence indicates that conditions favourable to marked vegetative growth of the pathogen tend to depress its virulence. The evidence available suggests that the new and vigorous hyphal growth necessary to sclerotia formation is initiated primarily by high relative humidity of the soil air.Further evidence was obtained that isolates of R. Solani pathogenic to potato may differ greatly in virulence and also in ability to form sclerotia, and also that the Corticium stage is an important source of pathogenic and sclerotia-bearing strains of this fungus. Finally, soil-grown inoculum 180 days old was as virulent as that freshly grown.