Erpocotyle microstoma (Brooks, 1934) Yamaguti, 1963, parasite of Sphyrna (Platysqualus) tudes (Valenciennes, 1822), is redescribed. E. schmitti sp. n., parasite of Mustelus schmitti Springer, 1939, is described. The systematic position of E. tudes (= Neoerpocotyle tudes) Cordero, 1944 and E. platensis (= N. platensis) Ma e Garzon et Holcman-Spector, 1968 is discussed, concluding that both species are synonyms of E. microstoma. According to the present study, the family Hexabothriidae Price, 1942, is represented in the Southwestern Atlantic Ocean by two genera: Callorhynchocotyle Suriano et Incorvaia, 1982 and Erpocotyle Van Beneden et Hesse, 1863, and by three species: C. marplatensis Suriano et Incorvaia, 1982, E. microstoma (Brooks) Yamaguti, 1963, and the herein proposed E. schmitti sp. n..