A new species of Gymnopus is described on the basis of collections from the subtropical cloud forest of eastern Mexico. Macro- and micromorphological characters, in combination with ITS sequences obtained from fruit body tissues, were used for its taxonomic circumscription. Basidiomata of this species were found growing scattered on fallen twigs of Quercus and also developing abundant long, black, wiry rhizomorphs. The authors discovered that these latter are used as part of nesting material by Myonectesoleaginous (Tyrannidae) inhabiting the subtropical cloud forest studied. A macro- and microscopical description as well as a discussion and illustrations are provided. A new combination in Gymnopus is proposed for Marasmiuswestii, a synonym of Marasmiusbrevipes.
Keywords: Omphalotaceae; Marasmioid fungi; Neotropical fungi; nesting biology.