Abstract
In order to examine the ecological relationships of metazoan parasites and their hosts, 63 specimens of Steindachnerina brevipinna have been collected from April to September 2006, in the rivers Guairacá and Corvo, tributaries of the low Paranapanema River. Five different parasite species have been found (Paranaella sp., Sphicterodiplostomum musculosum, Cosmoxynema vianai, Travnema travnema and Spinoxyuris sp.), with parasite richness from 1 to 4. The dominance index (C > 0.25) was calculated for S. musculosum and aggregation of S. musculosum and Paranaella sp. were reported. There were no associations or covariations between the species of parasites. Values did not show interference of parasite abundance at different gonadal maturity stages. The relative condition factor (Kn) did not show significant values regarding quantitative and qualitative data on parasitism. Statistical tests were significant between the prevalence and the abundance of parasites and the standard length of the hosts, as well as for the parasite abundance in different months and tributaries.