Many species of marine fish use coastal lagoons during early stages of their life cycles due to the protection provided by their turbid waters and complex structure of the environment, such as mangroves and mudflats, and the availability of food derived from the high productivity of these sites. In this study, we analyzed the diet of six species of juvenile marine fishes that use a karstic lagoon system in the northwest portion of the Yucatan Peninsula, Mexico. Through stomach contents analysis we determined the trophic differences among Caranx latus, Oligoplites saurus, Trachinotus falcatus, Synodus foetens, Lutjanus griseus, and Strongylura notata. C. latus, O. saurus, S. foetens, and S. notate, which are ichthyophagous species (>80% by number). L. griseus feeds mainly on crustaceans (>55%) and fish (35%), while T. falcatus feeds on mollusks (>50% bivalves, >35% gastropods). The analysis of similarities (ANOSIM) showed differences in the diet of all species. Cluster analysis, based on the Bray-Curtis similarity matrix revealed three groups; one characterized by the ichthyophagous guild (S. notata, S. foetens, C. latus, and O. saurus), other group formed by the crustacean consumers (L. griseus), and the third, composed by the mollusk feeder (T. falcatus). Species of the ichthyophagous guild showed overlap in their diets, which under conditions of low prey abundance may trigger competition, hence affecting juvenile stages of these marine species that use coastal lagoons to feed and grow.