Cretonomyia pristina new genus and new species, a fossil fly in amber from Cedar Lake, Manitoba, is described and assigned to the Ironomyiidae. This fossil establishes that the family, heretofore known from a single Australian species, Ironomyia nigromaculata White, existed during Mesozoic times in North America. Comparison of the extinct species with the living species shows that the family appeared little different 73 million years ago than it does today. In points of difference, the fossil species usually shows the more primitive conditions. It is postulated that the family arose in North America in late Jurassic – early Cretaceous times, dispersed to South America late in the Cretaceous Period and thence to Australia via Antarctica while the latter three were contiguous — 43 million years ago.