The primary purpose of this investigation is to present a monograph of the species of Gymnopilus known to occur in North America. The initial problem was to separate out the species of Gymnopilus from Pholiota and Flammula (sensu lato). This necessitated the preparation of a disposition list grouping the North American species of Pholiota and Flammula in their modern genera. This paper therefore contains two main phases: (1) a monograph of the North American species of Gymnopilus, and (2) a supplementary dispostion list grouping the North American species of Pholiota and Flammula in their modern genera. The disposition list had to be prepared first in order to determine which species were to be included in Gymnopilus. The procedure followed was to study type collections, original descriptions and correctly identified materials. Having thus segregated the species of Gymnopilus they were then described and grouped into a systematic arrangement on the basis of all the information which could be brought to bear on the problem. In addition to the above taxonomic studies, species of Pholiota and Gymnopilus have been cultured on a rice-sawdust medium to produce carpophores. The production of carpophores by this means made possible a comparative study of carpophore development in Gymnopilus praefloccosus and Pholiota adiposa. Another phase of this study involved animal tests on extract of Pholiota autumnalis and the correlation of these tests with certain collections of this species and Gymnopilus sapineus as an insight to the toxicology of the genus Gymnopilus. The results of this investigation are as follows: (1) a monograph of the North American species of Gymnopilus has been prepared; (2) a disposition list grouping the North American species of Pholiota and Flammula in their modern genera has also been prepared; (3) a method of culturing species of Pholiota and Gymnopilus for carpophore production was developed; (4) a comparative study of carpophore development in Pholiota adiposa and Gymnopilus praefloccosus was made; (5) negative results were obtained in regard to toxicity in animal tests on extract of Pholiota autumnalis and proof is presented that this species has been confused with Gymnopilus sapineus. Certain general conclusions can be drawn from these results. (1) There are 21 known species of Gymnopilus in North American and each of these is to be found in any region where suitable environment exists. (2) The North American species of Pholiota and Flammula are disposed among 14 modern genera, which indicates the heterogeneity of the two genera in the broad sense. (3) A mixture consisting of sawdust, brown rice, yeast extract and malt extract is a satisfactory culture medium for producing carpophores of lignicolous species of Pholiota and Gymnopilus. (4) From a comparative study of carpophore development, it is evident that Pholiota adiposa and Gymnopilus praefloccosus are annulate from the universal veil and that the partial veil is absent in these species. (5) From the negative results of animal tests on extract of Pholiota autumnalis and the correlation of these tests with certain collections of this species and Gymnopilus sapineus, it can be concluded that Pholiota autumnalis (Galerina marginata) is not poisonous and that cases of mushroom poisoning accredited to this species are really accreditable to Gymnopilus sapineus. In the monograph of the North American species of Gymnopilus, the following species are recognized: G. Harmoge, G. praefloccosus, G. palmicola, G. pholiotoides, G. Junonius, G. imperialis, G. sticticus, G. humicola, G. Abramsii, G. fulgens, G. Earlei, G. depressus, G. aureobrunneus, G. filiceus, G. areolatus, G. punctifolius, G. bellulus, G. picreus, G. liquiritiae, G. sapineus, and G. flavidellus.