None or very few sclerotia of Rhizoctonia solani (AG-1, IA) were formed on Hopkins agar medium without KH2PO4, though hyphae developed in some extent. While, the number and weight of sclerotia were decreased when MgSO4 was absent. As KH2PO4 in the basal medium was replaced with other phosphates different in cation, sclerotia were well formed. The optimal concentration of phosphate as KH2PO4 for the sclerotium formation, in number and weight, and hyphal development was in the region of 100ppm.The addition of magnesium resulted in the increase of the number and weight of sclerotia proportionally with the amount of the magnesium compound.The results indicate that phosphorus ion is indispensable to the sclerotium formation, whereas magnesium ion is promotive.The absence of phosphorus at the maturing stage resulted in the formation of few sclerotia, even though phosphorus was presented at the hyphal and initial phases. While defficiency of phosphorus in the early stage of sclerotium development showed no effect on the number and weight of sclerotia formed, if the phosphorus was supplied in subsequent periods. The branching internodes of hyphae grown on the phosphorus free medium were longer than those of on the medium containing phosphorus. This decrease of the hyphal branching will lead to the decrease of interweaving hyphae and will result in the scarce sclerotium formation.A large accumulation of 32P to sclerotia was strikingly observed in this experiment.