Nematodes of the cystidicolid Neoascarophis Machida, 1976, are all parasites of macrourid fishes, making up at present 5 species. Several other unidentified species have also been reported in several fish species from the northern and southern Atlantic Ocean, including 1 from Macrourus carinatus (Günther) (Macrouridae) in the southwest Atlantic Ocean. During a parasitological survey carried out on samples of M. carinatus from Patagonian waters, nematodes referable to Neascarophis were found in ulcers in the gastric mucosa. These nematodes Neascarophis sphaerocaudata n. sp. closely resemble N. macrouri by the posterior position of the vulva and the dilated posterior extremity in females. However, the new species differs from N. macrouri mainly by its larger size, a larger muscular esophagus, and a widely globose posterior extremity in females. SEM study of cephalic structures also showed morphological differences between both species, especially in the morphology of the submedian labia and lateral pseudolabia. In view of these differences a new species is proposed.