The author had the opportunity to examine therinid fishes of the species Xenomelaniris brasiliensis (Quoy & Gaimard), searching parasitic copepods. In this study various stages of larval development of Lernaeenicus longiventris Wilson were found. The crustacean embeded in tissues of the hosts, were quite similar to those of Gurney's larval stages found on the Sprat, with the only difference that these were youngest. Various stages of the copepodit development were observed during 1952-1954 at the laboratory of the Research Station of the Oceanographic Institute (Cananéia, southern coast of S. Paulo State, Brazil). These larvae were described and a comparison of the young and adult appendages was made. The resistence of the larvae was also considered. The author wishes to express his sincerest thanks to the National Research Council (Conselho Nacional de Pesquisas) of Rio de Janeiro, for a grant in aid.