The generic diagnosis of Aristocleidus Mueller, 1936 is emended based on a collection of 2 species of the genus from the gills of the Peruvian mojarra, Diapterus peruvianus (Gerreidae), from a lagoon along the Pacifi c Coast of Mexico. The genus is characterized in part by species having a deeply incised base of the ventral anchor with the tip of the superfi cial root conspicuously directed toward that of the deep root. Aristocleidus hastatus Mueller, 1936 is redescribed and Aristocleidus lamothei n. sp. is described from D. peruvianus from the Tres Palos Lagoon, Guerrero State, Mexico. The fi nding of A. hastatus on D. peruvianus in Mexico represents new host and locality records for the species.