New Zealand's first pre-Pleistocene mystacinid bat fossils have been recovered from early Miocene sediments of the Manuherikia Group near St. Bathans, Central Otago. Mystacinidae, which belongs to the Gondwanan bat superfamily Noctilionoidea, is the only living mammalian family endemic to New Zealand, although its distribution included Australia in at least the Oligo-Miocene. The only member of the family definitely surviving is the peculiar walking bat Mystacina tuberculata. The St. Bathans mystacinid fossils consist of isolated teeth and postcranial fragments that appear to represent two new taxa of similar size and functional morphology (dental and wing) to Quaternary mystacinids. They suggest an Australasian mystacinid radiation now numbering at least eight species: four from New Zealand and four from Australia. The St. Bathans fossils demonstrate that mystacinids have been in New Zealand for at least 19-16 Ma and signal the longest fossil record for an endemic lineage of island bats anywhere in the world. They add to the list of endemic vertebrate lineages present in Zealandia by the early Miocene, including leiopelmatid frogs, sphenodontids, acanthisittid wrens, adzebills, moa, and kiwi.