Five new species of Potamotrygonocotyle (Monocotylidae) are described and Paraheteronchocotyle amazonense Mayes, Brooks et Thorson, 1981 (Hexabothriidae) is redescribed from monogenoideans collected on the gills of species of Potamotrygonidae from the Negro River, Amazon, Brazil. Potamotrygonocotyle quadracotyle sp. n. and P. umbella sp. n. were found parasitizing an undescribed species of Potamotrygon; Potamotrygonocotyle rarum sp. n. is described from Potamotrygon schroederi; Potamotrygonocotyle rionegrense inhabits Potamotrygon cf. motoro; Potamotrygonocotyle aramasae sp. n. is a parasite of Paratrygon aiereba; and Paraheteronchocotyle amazonense is reported from Potamotrygon orbignyi. The diagnosis of Paraheteronchocotyle is emended, and P. amazonense is redescribed.