A new oxyuroid nematode, Ichthyouris nunani n. sp., is described from the intestine of the freshwater benthopelagic characiform fishes Laemolyta taeniata and Curimata acutirostris, both of which were caught in the Tocantins River, State of Maranhão, Brazil. The new species is easily distinguished from all other species of the genus mainly because of the distribution of the male caudal papillae, the morphology of the spicule, and the fact that the eggs have an operculum and many long, thread-like filaments at each pole. This is the first helminth species described in both hosts, and this report contributes to the knowledge of the helminthological fauna of fish in the Neotropical region.