Gymnopilus is a genus of gilled mushrooms within the fungal family Cortinariaceae containing over 150 rusty-orange spored mushroom species formerly divided among Pholiota and the defunct genus Flammula. The fruiting body is typically reddish brown to rusty orange to yellow, medium to large, often with a well developed veil. Most members of Gymnopilus grow on wood but at times may appear terrestrial if the wood is buried or decomposed. Members of Pholiota and Cortinarius are easy to confuse with Gymnopilus. Pholiota can be distinguished by its viscid cap and duller (brown to cinnamon brown) spores, and Cortinarius grows on the ground. Beginners can confuse Gymnopilus with Galerina, which contains deadly poisonous species.
14 members of Gymnopilus contain psilocybin , although their bitter taste often deters recreational users. These species include Gymnopilus aeruginosus, Gymnopilus braendlei, Gymnopilus intermedius, Gymnopilus junonius, Gymnopilus liquiritiae, Gymnopilus luteofolius, Gymnopilus luteoviridis, Gymnopilus lutes, Gymnopilus purpuratus, Gymnopilus sapineus, Gymnopilus subpurpuratus, Gymnopilus validipes and Gymnopilus viridans. Subspecies of Gymnopilus spectabilis from Japan are reported to contain psilocybin, while some western North American members are inactive.
Several species of Gymnopilus contain bis-noryangonin [4-hydroxy-6-(4-hydrostyryl)-2-pyrone] and hispidine [4-hydroxy-6-(3,4-dihydroxystyryl)-2-pyrone], which are closely related to the alpha-pyrones found in kava.