A total of 8 specimens of Urophycis brasiliensis (Kaup, 1858) from waters off Mar del Plata, Argentina (38°08′S, 57°32′W), were examined for parasitic nematodes. A new nematode species, Cucullanus bonaerensis n. sp., is described (prevalence 50%, mean intensity 15.5). The new species differs from its congeners in the distribution pattern of caudal papillae (particularly fourth pair) and phasmids, length of the body and spicules, and by the position of the excretory pore (in the posterior third of esophagus). Species of Cucullanus, reported previously as C. heterochrous Rudolphi, 1812, and Cucullanus sp. from U. brasiliensis from Puerto Quequén, Argentina (38°37′S, 58°53′W), are considered as members of the new species.