Nematode infection indices were recorded in Lisa, Mugil incilis, from Cartagena Bay and Totumo Marsh, north of Colombia, during an 8-mo period (February to September 2002). Parasite prevalence (74.67% vs. 53.48%), intensity (9.73 +/- 2.98 vs. 2.73 +/- 1.07), and abundance (7.49 +/- 2.21 vs. 2.04 +/- 1.17) were significantly greater in Cartagena Bay, an estuary polluted by domestic sewage and industrial discharge, compared with those of Totumo Marsh, a body of water with low levels of contamination. Parasites were found below the spine, within the liver and the intestinal mesenteries, and a small percentage in muscle. Morphological analysis of nematodes indicated the presence of the third larval stage of several species belonging to the Anisakidae. Size correlated moderately and significantly with parasite intensity in fish collected from Totumo Marsh (R = 0.336; P < 0.001); in Cartagena Bay the correlation was also significant, but low and negative (R = -0.212; P = 0.003), clearly showing differences in host-parasite ecology. Fish health status, as represented by condition factor and hepatosomatic index, did not show any correlation with the parasite prevalence in fishes collected in either sampling areas. These results suggest, for the first time, that the consumption of Lisa from the Atlantic coast of Colombia could represent a risk for human infection.