Ergasilus yumarikus sp. n. (Copepoda, Poecilostomatoida, Ergasilidae) is proposed. The specimens were collected from the gill filaments of the freshwater fishes Pygocentrus nattererì, Serrasalmus rhombeus and Pristobrycon eigenmanni from Guaporé River near Pimenteiras, Guaporé and Mamoré Rivers, near Surpresa, Jiparaná River near Jiparaná and Pacaás Novos River near Guajará-Mirim, Rondônia state, Brazil. The new species has a falciform pectinate seta on the first exopod that indicates relationship to six previously described Amazonian species. This species differs from others in the shape of body, ornamentation of the legs, antennas and urosomites.
Copepoda; Ergasilus yumarikus sp. n; Amazon; Serrasalmidae; Rondônia