Several Ponto-Caspian gobiids have expanded from their native distribution ranges to Europe and North America. Asknowledge on their bio-ecological features in their native range is still limited, the trophic ecology of monkey goby Neogobiusfluviatilis and Western tubenose goby Proterorhinus semilunaris was studied in three natural lakes in the Marmara Region ofNW Turkey using the stable isotopes of δ13C and δ15N. In two of the lakes, the trophic niches (as the isotopic niche) of thegobies were highly divergent with co-existing native fishes, with no overlap. Moreover, mixing models suggests considerableinter-specific dietary differences. In all lakes, the trophic niches of gobies were never significantly larger than those of coexistingfishes. These results suggest that when introduced outside of their natural range, the gobies might integrate into newfish communities via exploiting resources that are underexploited by native fishes or will initially share resources with thesespecies before their niches diverge, perhaps through competitive displacement.