The adult and pupa of Trichothaumalea japonica sp. n. are described from Japan. This widespread relict genus also includes three species from western [T. pluvialis (Dyar & Shannon), T. pilosa (Garrett)] and eastern (T. elakalensis Sinclair) North America. T. japonica + T. elakalensis are most closely related and together are the sister-group to the western Nearctic species group. Examples of Diptera with similar biogeographic patterns are listed. In addition, male genitalic homologies in this genus are re-examined.