ABSTRACTMarasmius inaquosi sp. nov.was isolated from palo verde wood collected inside a woodrat midden in the Arizona Sonoran Desert and is described from basidiomata formed in vitro. The species belongs in sect.Sicci, has an amphithallic mating system, and is the only knownMarasmiusfrom desert environments. Woodrat middens are mesic microhabitats favorable for fungal growth in an otherwise xeric environment. Woodrats may play a role in dispersingM. inaquosito suitable microhabitats. The species is illustrated and compared with phenetically similar taxa, and details on culture morphology are provided.