Trypanosoma sp. (Sarcomastigophora: Kinetoplastida) was found in the blood of the Korean catfish, Silurus asotus, for the first time in Korea. The morphological characteristics of Trypanosoma sp. in the present study were similar with those of T. carassii, T. tincae and T. danilewskyi. However, the free flagellum length of Trypanosoma sp. was obviously shorter than that of those species. The species identification was reserved until elucidating the pleomor-phism according to the phase of infection and the cross infectivity among fish species.