In this study the structure and diversity of the endohelminth community, as well as its interactions with 73 specimens of Piaractus mesopotamicus are described. The fish were caught in Aquidauana and Miranda rivers, both located in the Pantantal region, in the state of Mato Grosso do Sul. A total of 608,788 specimens of helminthes were found, representing five species: Dadaytrema oxycephala, Chabaudinema americanum, Spectatus spectatus, Rondonia rondoni and Echinorhynchus jucundum. D.

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... was more prevalent (91.78%) followed by R. rondoni (73.97%). Simpson's index indicated dominance in the endohelminth infracommunities (C = 0.396) and D. oxycephala was considered central species. A total of 54 hosts had between 2 and 3 species of helminthes. The pairs of the species R. rondoni/D. oxycephala and S. spectatus/D. oxycephala showed significant positive correlation between abundances. There was a significant positive correlation between total length and abundance of S. spectatus (rs = -0.2467 and p = 0.0353). Abundance of R. rondoni between the periods of 1/2003 and 2/2003 (p = 0.0356) and 1/2003 and 4/2004 (p = 0.0009) was significantly different. There was significant prevalence of E. jucundum in the female hosts.