This work deals with Bomolochus xenomelanirisi n.sp. - Copepoda Cyclopoida - that was found as parasite on Xenomelaniris brasiliensis (Quoy & Gaimard), a fish inhabiting the south seashore region of São Paulo State (Brazil). On examining 100 fishes, the author found 30 ♀ ♀ and 14 ♂ ♂ , all of which were attached to the internal portion of the posterior region of the opérele. The new species here described is related to Bomolochus nitidus and B.exilipes, both described by Wilson, but differs from them by the shape of the maxilliped, by the greater number of setae of the first maxilla, by the lenght of the second antennae, by the strongest armature of the first antennae and by the shape and number of the eggs contained in the ovigerous sacs. The author describes male and female, illustrating the descriptions with 21 drawings. Some ecological notes are presented. The author is indebted to the National Research Council (Conselho Nacional de Pesquisas) of Rio de Janeiro, for a grant in aid, for which thanks are due.