This report is the first confirmed record of ciliatosis in North American black flies. A new species, Tetrahymena rotunda n. sp. is diagnosed as a parasite of Simulium spp. This species is characterized by its globose shape, large size and total kinety number, and relatively small buccal cavity. In a five-year period (1976-1980) in upper New York State, the infection rate in Simulium spp. parasitized by this tetrahymenid was low, usually \<2\%. Ciliates were found in Simulium tuberosum larvae and in the larval, pupal, and adult stages of Simulium venustum. Hosts typically harbored ~10 ciliates, a number far less than generally reported for ciliate-infected dipterans.