Diplostomum (Austrodiplostomum) compactum (Lutz, 1928) metacercariae (Platyhelminthes, Digenea) were found in six fish species, belonging to two orders (Characiformes and Perciformes) and three families (Erythrinidae, Sciaenidae and Cichlidae). A total of 477 individuals were collected, from August 1999 to May 2001, in the upper Paraná River floodplain, Brazil. The metacercariae were infecting the eyes of six host species and the brain of five of them. Plagioscion squamosissimus, Satanoperca pappaterra and Cichla monoculus presented the highest values of prevalence, mean intensity and mean abundance. Diplostomum (A.) compactum was over-dispersed in the population of Hoplias aff. malabaricus, S. pappaterra, P. squamosissimus, Crenicichla britskii and C. monoculus. No significant difference was observed in the metacercariae distribution in the right and left eyes. The abundance of D. (A.) compactum metacercariae was positively correlated with the host's relative condition factor in H. aff. malabaricus and S. pappaterra. No significant difference of the abundance of parasitism between the sex of the hosts was evidenced. In relation to the prevalence, significant difference was observed only in C. britskii. Positive correlations were verified between the prevalence of infection and the standard length of the specimens in C. britskii and the intensity of infection and the standard length of the hosts in P. squamosissimus and C. monoculus. In relation to parasite abundance and the standard length class of the host, significant differences were observed in P. squamosissimus, C. britskii and C. monoculus.

Differential white blood cell counts from Schizodon intermedius infested by Lernaea cyprinacea were carried out and compared using the Wilcoxon matched pairs test. The observations were performed in fish infested by 16-77 adult Lernaea, fish with parasitic lesions but without attached crustaceans, and non parasitized fish (control group). The specimens of Schizodon intermedius were obtained from the University of Londrina fish farming facilities. The following leucocytes were observed: lymphocytes, neutrophils, monocytes, basophils, eosinophils and immature leucocytes. Intense lymphocytopenia and neutrophilia were observed in the infested hosts. Consistent increasing of monocyte percentage values occurred in the infested fish. The highest values for immature leucocytes counts were recorded from infested fish specimens.