It is described Trypanosoma guairaensis sp. n. parasitizing Megaloancistrus aculeatus taken from Paraná river (Itaipu reservoir). Morphometric characters such as length and width of the nucleus, length of the free flagellum and nuclear index values show that T. guairaensis is different from all the trypanosome species so far described for fresh water fishes from Brazil.
Trypanosoma guairaensis; Megaloancistrus aculeatus; Paraná river; Itaipu reservoir; Brazil