The fish fauna in the State of Acre represents 10·7% of all fish species recorded from Brazil, but, despite this, there are few fish parasite studies in this area. The recent expansion of fish farming in Acre prompted a need for helminthological studies of the most commonly consumed fish species in the area, Pimelodus blochii (Pimelodidae). The aim of this study was to analyse the helminth fauna of P. blochii from the Acre and Xapuri Rivers in Northwestern Brazil. Numerous nematodes were collected from the intestine and two species of the family Atractidae were identified: Rondonia rondoni Travassos, 1920 and Orientatractis moraveci n. sp. The new species is distinguished from its congeners mainly by having: 10 pairs of caudal papillae (3 pairs pre-cloacal, 2 pairs ad-cloacal and 5 pairs post-cloacal); unequal spicules of 161-198 and 69-100 µ m long; and a gubernaculum 38-58 µ m long with an antero-lateral process. Morphological and ultrastructural data on O. moraveci n. sp. and R. rondoni are presented, in addition to new genetic data based on partial 18S rDNA and 28S rDNA. The taxonomic status of Labeonema synodontisi (Vassiliadès, 1973) is discussed, suggesting that it should be returned to the genus Raillietnema.