Carbon and nitrogen stable isotope analyses were carried out to determine the diet composition and trophic position of the invasive species Carassius gibeho to help understand the potential impact of this species on the native fish fauna in the Karamenderes River, northwest Turkey. Filamentous algae were the most important diet component according to stable isotope mixing models. Filamentous algae and detritus formed also considerable part of the gut contents. The trophic position of C. gibeho was 2.43 +/- 0.5, 2.04 +/- 0.4 and 3.35 +/- 0.5 estimated by three different methods. The trophic niche width of this invasive species was larger than those of the native species. Our results indicate that the high dietary plasticity of C. gibelio and its lower trophic position than the other fish community members, can contribute to its success as an invasive species.