Abstract 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.