Allen's big-eared bat, Idionycteris phyllotis, is one of the most poorly known bat species in North America. It was not discovered in the United States until 1955, and it was not known from Utah until 1969. The species is rare in Utah, occurring only in the southern portion of the state.
Preferred habitats for the species include rocky and riparian areas in woodland and scrubland regions. Little is known about the breeding activity of the species, but females have been found with single young during the late spring and early summer. Allen's big-eared bat is an insectivore, eating insects captured in flight or plucked from vegetation. The species is nocturnal, roosting in caves or rock crevices during the day.