Descripción: Neoechinorhynchus (Hebesoma) colastinense sp. n. is described from the intestine of Pachyurus bonariensis a freshwater fish from Parana River basin. The new species is characterized by having a cylindrical trunk, elongated but swollen anteriorly; a spherical proboscis with a prominent apical organ; the anterior circle of hooks very large, alternating in 2 levels, separated from more posterior circles of hooks, but sometimes surpassing the hooks of the middle and posterior circles; a relatively long neck; the male reproductive system occupying 78-81% (79%) of the trunk length; the female reproductive system occupying 27-39% (32%) of the trunk length; and eggs elongated with polar prolongation of the fertilization membrane. Members of the species can be distinguished easily from the other species of South American Neoechinorhynchus Stiles & Hassall, 1905 because they are the only ones with characteristics of the subgenus Hebesoma. Additionally, two other species are recorded for the first time in Argentina: Neoechinorhynchus {Neoechinorhynchus) macro-nucleatus Machado Filho, 1954 from Lycengraulis grossidens and N. (N) pimelodi Brasil-Sato & Pavanelli, 1998 from several species of Pimelodus. The presence of an apical organ at the proboscis tip is recorded in the new species and N. (N.) macronucleatus. Studies using scanning electron microscopy revealed the presence of non-rimmed pores in the tegument throughout the trunk, neck and proboscis of all three species. A key to the South American species of Neoechinorhynchus is provided.