A detailed comparison of the gross morphology of hyphae, monilioid cells, and sclerotial formation and structure of 17 isolates of species in seven genera of theRhizoctoniacomplex were studied. Cytochemistry of both hyphal and sclerotial cells was also studied. Gross morphology of hyphae was similar in many of the isolates and was considered of limited taxonomic value. Sclerotial formation in species ofThanatephorus,Waitea,Aquathanatephorus, and possiblyCeratobasidiumwas of the monilioid cell type. That ofAtheliawas of the hyphal strand type. Although initially similar, mature sclerotia in each of the genera showed great diversity. Most sclerotia ofThanatephoruswere constructed of loosely arranged monilioid cells. Those inCeratobasidiumdeveloped a marginal rind;Aquathanatephorus, a pitted surface and hyphal mat;Waitea, a bilaminate condition with elongate marginal and globose medullary cells; andAthelia, a complex tetralaminate structure. Distribution and concentration of carbohydrates, proteins, and lipids differed in the various genera. The strong dextrinoid reaction of the cytoplasm ofThanatephorusandAquathanatephorus, accompanied by higher total lipids in cell walls and septa, was of significance. Stronger reactions for cellulose were found in species ofCeratobasidium, while cellulose appeared more concentrated in sclerotial cells than in hyphal cells ofThanatephorus. Lipids were deposited more abundantly in sclerotial cells than in hyphal cells.