Summary1.Hymenochaete rubiginosa(Schrad.) Lév. is a common saprophyte on decorticated chestnut about Ithaca; it is found more rarely on oak.2.The fruit bodies are annual and quite xerophytic; spores are shed intermittently during moist periods for several months.3.Spore germination occurs best in decoctions of oak or chestnut sawdust and tap water. Mycelial growth was restricted and clamp connections were not observed.4.The first evidence of decay in oak and chestnut consists in the formation of white spots here and there in the wood. Cavities lined with cellulose are formed through the partial or complete digestion of the elements.5.The tissues between the infected areas remain nearly or quite as sound as in normal wood. Rarely do the pockets coalesce through the digestion of intervening tissue.6.The chemical action of the fungus consists first in the delignification of the elements attacked. This begins with the tertiary layer and continues outward.7.Soon after the middle lamella is attacked it is dissolved and the elements separate or remain loosely attached at the corners.8.Cellulose digestion continues after the elements become isolated. The thin-walled elements including pith ray cells and wood parenchyma are first dissolved.9.The pockets arising in the wood are at first lined with partially digested elements which consist of pure cellulose. In the final stage the white lining entirely disappears.10.The decay accompanying the fungus is comparable to that caused by Trametes abietis Karst., on red spruce and other conifers. It has a remarkable resemblance in superficial appearance and method of attack to that caused byStereum frustulosum(Pers.) Fr.11.A superficial, peripheral type of decay, in which all the elements are attacked but not entirely digested, usually accompanies the typical decay caused byHymenochaete rubiginosa.The walls remaining are dark in color and fail to respond to the cellulose reaction.