One hundred and ten specimens of Pseudoplatystoma corruscans (Agassiz, 1829) (Pimelodidae), "pintado", and 582 specimens of Schizodon borelli (Boulenger, 1900) (Anostomidae), "piava", collected in the floodplain of the high Paraná River were analyzed. P. corruscans presented 74.54% of parasitism while S. borelli presented 19.42%. Results of the present research show that in the case of P. corruscans, two species of proteocephalideans presented a significant relationship with the type of environment with regard to prevalence. Spasskyelina spinulifera (Woodland, 1935) was the only species that presented a relationship between the intensity of infection and the environment. In S. borelli only Cucullanus pinnai Travassos. Artigas & Pereira, 1928 presented a relationship with the environment. In this case the mean intensity was greater in lentic environments. The high values of the similarity index in the three environments studied indicate a considerable homogeneity of the endohelminths' species with regard to P. corruscans and S. borelli. This is probably due to what occurs with environments during the high water period when they are interlinked. The hydrological level was a less influential variable in endoparasitic infrapopulations of the hosts studied. In P. corruscans only three species, S. spinulifera, Nomimoscolex sudobim Woodland, 1934 and Megatnylacus travassosi Pavanelli & Rego, 1992 presented higher mean intensities in the high water period. This fact was also observed in the case of C. pinnai in S. borelli.

In the present paper the authors describe and illustrate a new species of proteocephalidean cestode, Monticellia belavistensis, sp.n., parasite of a freshwater siluriform fish (thorny catfish). Pterodoras granulosus (Valenciennes, 1833), from the Itaipu Reservoir and Paraná River, in the region of Porto Rico. This is the first reference of a proteocephalid in this host. The values of prevalence and mean intensity of infection are given and the life cycle is discussed.