“Brine flies” or “salt flies” of the genus Ephydra Fallén are often very abundant in coastal salt marshes, salt and alkaline lakes and ponds, and around thermal and mineral springs. From a detailed study of the male genitalia, 16 species of Ephydra are recognized in North America. These are classified in 3 subgenera: Ephydra s. str., with 6 species in the riparia group and 7 species in the glauca group; HALEPHYDRA, new subgenus with 1 species; and Hydropyrus Cresson with 2 species. Characters of the male genitalia of each species are described and illustrated, brief diagnoses are given of external features, and a key is presented for the recognition of all taxa. The known biology and distribution are summarized. Ephydragoedeni, n. sp. is described from hot springs in Oregon and nearby states; E. packardi, new name is proposed for E. halophila Packard (not von Heyden), and E. currani, new name for E. salina Curran (not von Heyden); and the correct application of other specific names is indicated from a study of the respective types.