RESUMEN
Monogenoids are ectoparasites that have a simple one-host lifecycle, high species diversity, and a relatively high host specificity. During studies on the helminth fauna of fishes from the Jurua River, in Acre State, Brazil, a new species of the monotypic genus Unibarra Suriano & Incorvaia, 1995 was found parasitizing Oxydoras niger Valenciennes, 1821. Unibarra juruaensis n. sp. is allocated in the genus based on the presence of a single haptoral bar, marginal hooks similar in shape and size, gonads partially overlapping, and a conspicuous filament which connects the base of the male copulatory organ with the accessory piece. The new species differs from the only species of the genus by the smaller size of the body and of the structures, by the morphology of copulatory complex, with an accessory piece thinner than that of U. paranoplatensis Suriano & Incorvaia, 1995 and by the presence of two eyespots. The type species, U. paranoplatensis, is referred in a new host, Pimelodus blochii Valenciennes, 1840, with new morphological data. A table of measurements of the new species and previous and the present reports of U. paranoplatensis is presented.