| Summary: | The present study describes three new species of monogeneans parasitizing the gills of anostomid fishes from the Upper Paraná River basin, southeastern Brazil: <i>Jainus beccus</i> n. sp. and <i>Jainus radixelongatus</i> n. sp. on <i>Leporinus friderici</i>, <i>Leporinus octofasciatus</i>, <i>Leporinus striatus</i>, and <i>Megaleporinus elongatus</i>; and <i>Jainus ornatus</i> n. sp. on <i>L. friderici</i>. The new species differ from other congeners by the morphology of the accessory piece. There is a semicircular distal portion resembling a “bird’s beak” in <i>Jainus beccus</i> n. sp. It composed of two subunits—one ventral and more sclerotized, sickle-shaped, and another dorsal with three projections—in <i>Jainus radixelongatus</i> n. sp. There are two elongated and sclerotized subunits, both of which have a sickle-shaped distal portion, in <i>Jainus ornatus</i> n. sp. Supplementary observations not reported in the original descriptions of the type-species <i>Jainus piava</i> Karling, Bellay, Takemoto & Pavanelli, 2011 are proposed, as follows: the presence of a thin and delicate ventral bar, which can vary greatly in shape; an accessory piece not articulated with the MCO’s base. This paper provides the first phylogenetic study based on LSU rDNA and COI mtDNA gene sequences for <i>Jainus</i>, improving and clarifying the understanding of host–parasite relationships in neotropical characiforms.
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