One hundred and ten specimens of Pseudoplatystoma
corruscans (Pimelodidae) and 582 specimens of Schizodon
borelli (Anostomidae) collected in the high Parana River
were analyzed. On necropsy 74% of P. corruscans were
found to be parasitized; proteocephalidean cestodes presented
the greatest number. With regard to S. borelli, the
percentage of parasitism reached 19.42% and the nematode
Cucullanus pinnai was the most abundant. The absence of
correlation between the endoparasitic diversity and the
standard length of the two host species indicates that each
one presents homogeneity in alimentary behaviour during all
its life time, permiting the uniform recruitment of the same
species of endoparasites during all its ontogenetic
development. Independence of diversity values in relation to
sex of P. corruscans and S. borelli evidences
that the ecological relationships are similar between males
and females in these species. Both host's infrapopulations
presented a typical overdispersed pattern of distribution with
isolationist characteristics.
