On the basis of a survey of the literature and of test results here reported on more than 400 fungus isolates, cellulose-decomposing activity is shown to be very widely distributed but not universal among the fungi. It occurs with a high degree of regularity among isolates of certain genera—e.g., Alternaria, Chaetomium, Curvularia, Fusarium, Gliomastix, Helminthosporium, Humicola, Memmoniella, Stachybotrys, and Trichoderma, and probably with equal frequency in other genera which have not been so extensively studied. It occurs in a greater or smaller number of species in many of the several groups or subdivisions of the genus Asoergillus in the genus Penicillium, apparently not at all in Mucor, Rhizopus, and other genera of the Mucoracae, and probably only rarely among the actinomycetes. Tests on Cladosporium herbarum show that the temperature optima for growth of isolates of this fungus are in general distinctly below the approximately 30 degrees C used by several other workers in experiments with it; this fact is believed to account, at least in part, for the poor cellulose-decomposing activity shown by it in their tests. Tests on Pullularia pullulans have failed to detect cellulytic activity.