THE filamentous fungi have been called ubiquitous and omnivorous, and certainly they are widely distributed in Nature where they play a most important part in the decomposition of complex organic matter and as pathogens for many plants. Since much of the plant-derived organic matter in Nature contains a large amount of pentoses, mostly in the form of pentosans, a study of the pentose metabolism of the fungi which decompose this material is of interest not only because it is a basic phenomenon, but also, as we have found, because it is somewhat unique.