Spectatus spectatus Travassos, 1923 (Nematoda: Kathlaniidae) found in the intestine of Piaractus mesopotamicus (Holmberg, 1887) (Characiformes: Serrasalmidae) from the River Miranda, Mato Grosso do Sul, Brazil is redescribed based on morphological evaluation of newly collected material and examination of type and voucher specimens from the Coleção Helmintológica do Instituto Oswaldo Cruz. The following characteristics previously unreported or insufficiently described were observed: morphology of lips, presence of lamellae-like supplementary lips, presence of pharynx and cuticular ring surrounding the oral opening associated with a complex cuticular apparatus anterior to the pharynx, the number and arrangement of caudal papillae (13 pairs plus 1 unpaired), and the position of nerve ring. Since S. spectatus is the type species of Spectatus, the diagnosis of this Neotropical genus is amended. Synonymy of Chabaudinema Díaz-Ungría, 1968 with Spectatus, first proposed in 1980 by Baker, is supported by the present data. Molecular data that include the first sequence of the SSU rDNA for any species of Spectatus indicate a basal position of S. spectatus within Cosmocercoidea, forming a distant lineage from that comprising 2 species of Falcaustra Lane, 1915. This separate position of S. spectatus supports validity of the genus.

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