A new species of Hysterothylacium is described and figured. The
nematodes were collected from the intestine of a neotropical
catfish, Rhamdia sapo, collected from its southernmost locality
(Naposta stream and Sauce Grande river, Buenos Aires Province).
The specimens resemble H. murrayense but differ in having shorter
spicules, in the number and distribution of papillae and the
relative size of intestinal caecum and ventricular appendix.
