Nematode infection indices were recorded in Hoplias malabaricus captured in six different rivers and a marsh belonging to the North Coast Basin of Colombia, and from the Amazon River, during February 2003-December 2004. Preliminary morphological analysis of nematodes indicated the presence of Contracaecum sp. Parasites were mostly found in the intestinal mesenteries and a very low percentage in muscle. Parasite prevalence in all sampling locations at the north of Colombia was 100%, whereas in the Amazon River it was 6.12%. The mean intensity in the different stations were as follows: Magdalena River at the City of Magangué (58.92+/-7.59), Magdalena river at the city of Zambrano (128.9+/-7.08), Sinú River (53.88+/-4.92), Dique Channel (207.3+/-59.52), Cauca River (77.26+/-9.35), Atrato River (21.11+/-2.6), San Jorge River (39.5+/-7.13), and Totumo Marsh (62.5+/-6.38). In average, all specimens of Hoplias malabaricus from the north coast basin of Colombia were infected with a mean intensity of 77.82+/-4.81 (1-466 parasites per host) whereas in fish from the Amazon River this value was significantly lower (intensity 1.0+/-0.0). Size and weight correlated significantly with parasite intensity in fish collected from sampling locations at the north of Colombia (R=0.240, P<0.001 and R=0.199, P=0.008, respectively). Moreover, a significant, but low and negative correlation was found between condition factor and parasite intensity (R=-0.159, P=0.034), suggesting a possible impact of parasites on fish health. These results suggest, for the first time, that the parasitism in Moncholo is a widespread phenomenon in Colombian rivers and could represent a risk factor for human consumers.