Two new species of Dactylogyridae (Monogenoidea) found parasitizing the gills of the pacu, Piaractus mesopotamicus (Holmberg, 1887), are described: Anacanthorus toledoensis n. sp. and Mymarothecium ianwhittingtoni n. sp. These monogenoideans were collected in the gills of all 38 pacus reared in cages in Toledo, State of Paraná, Brazil. Anacanthorus toledoensis n. sp. is characterized by the slightly sigmoid male copulatory organ (MCO) with a membranous flap along the longitudinal axis, which ends at the distal, spatulated portion of the organ, and by the rod-shaped accessory piece bearing a protruding median projection. Mymarothecium ianwhittingtoni n. sp. differs from all other species of the genus by the morphology of the anchors and by the structure of the copulatory complex comprising an arcuate MCO, with basal flap and accessory piece with a distal rod, somewhat sigmoid, and subterminal flap hooked.