Lernanthropus cynoscicola sp.n. (Copepoda: Lernanthropidae), a parasite of sciaenid fish Cynoscion striatus Cuvier from the coast of Argentina, is described and illustrated. The new species resembles five other species of Lernanthropus in general body shape, but it can be distinguished from them by the relatively short and tapered dorsal plate of the 4th leg-bearing segment, the length of the 4th legs being approximately equal to the rest of the body, and other differences. L. trachuri Brian, 1903 is reported for the first time in Argentina on Trachurus lathami from the coast of the Buenos Aires province.