Abstract
A new species of dactylogyrid monogenean, Trinibaculum rotundus n.sp. is described and illustrated from the gills of freshwater fish Schizodon borellii (Boulenger, 1900) from the upper Paraná River floodplain, Brazil. Trinibaculum rotundus n.sp. has vagina dextrolateral slightly sclerotised ending at level of seminal receptacle; copulatory organ coiled with 1.5 clockwise rings; accessory piece not articulated; body disk-shape with absence of haptor peduncle and three bars (one ventral bar and two dorsal bars).