A new genus and three new species of the microchiropteran family Mystacinidae are described from Miocene freshwater limestones in northern Australia. The type species, Icarops breviceps new genus and species, is from the middle Miocene Bullock Creek deposit, Northern Territory; I. aenae new species and I. paradox new species are from the slightly older (early Miocene) Wayne's Wok and Neville's Garden Sites at Riversleigh, northwestern Queensland. Fossil mystacinids are rare in each deposit and represented so far only by lower teeth and dentary fragments. They are characterized by a suite of apomorphies shared only with Quaternary mystacinids endemic to New Zealand. The family Mystacinidae has no pre-Pleistocene record and its relationships to other groups of bats remain unclear. Possible sister-groups include South American noctilionoids and the cosmopolitan molossoids and/or vespertilionoids. The presence of plesiomorphic mystacinids in the Australian Tertiary suggests an Australian origin for the family.