During 1952-53, while studying the parasitic copepods of some brazilian fishes, at the laboratory of the Research Station of the Oceanographie Institute (Cananéia, southern coast of S. Paulo State. Brazil), the author found 87 copepods of the genus Lernaeenicus, 176 of the genus Fomolochus and 2.093 of the genus Ereasilus. These copepods were obtained from the gills of the mugilid fish Xenomelaniris brasiliensis (Q & G.). In this paper the author deals only with specimens of the genus Ergasilus since the other two genera, Lernaeenicus (Carvalho 1953, p. 191-190) and Bomolochus (Carvalho 1955, p. 143-151) were already studied previously. The species now described, according to the key of Wilson (1911, p.327) is related to sieboldii, lizae, nanus and versicolor, but differs from them by various important characters. A diagnosis of the new species is presented and comparisons are made with the related ones. Some considerations concerning the parasitism of the young fishes by leaches probably belonging to the family Ichthyobdellidae are presented, as well as references about two cases of parasitism by isopods and a few laboratory experiments related to the study of the nauplius stages of Lernaeenicus. The author is indebted, with due thanks, to the National Research Council (Conselho Nacional de Fesquisas) of Rio de Janeiro, for a grant in aid.
