A new, monomorphic trypanosome, Trypanosoma magdalenae sp. n. was found in five of 38 fish, Petenia kraussii, from the Rio Magdalena in Colombia, South America. It is the first trypanosome species designated from freshwater teleosts in Colombia. The trypomastigotes measured in 42.4 micron +/- 2.05 SD (range, 39.8 - 45.6) by 2.1 micron +/- 0.20 (2.0 - 2.5). Their nuclear index was 1.6 micron +/- 0.22 (1.25 - 1.84) and their kinetoplastic index equaled 1.1 micron +/- 0.02 (1.08 - 1.15). Infection intensity was usually limited to two or three flagellates per 40 microliter of packed blood cells.