A study of decay of black cotton wood, Populus trichocarpa Torr. and Gray, revealed that, although 70 species of fungi caused decay, only six caused significant loss in living trees and that two of these, Polyporus delectans Peck and Pholiota destruens (Brand.) Quél., caused 92% of this loss. The sporophores and associated decays of certain of the fungi are described and their relative importance is indicated. Data on the relation of decay to tree age showed that, despite a high incidence of infection, the average volume of decay per infected tree was low. In general, decay proved important only to the recovery of specialty products such as plywood. A significant reduction of strength was found to occur in wood containing an early stage of decay caused by P. destruens. The practicability of segregating trees having a high or low probability of being decayed was demonstrated through the use of decay indicators.