In temperate fish polyculture, common carp (Cyprinus carpio) and roach (Rutilus rutilus) are two fish species commonly reared in the same ponds. In the natural environment, these two species are considered omnivorous and may compete for food sources. However, few is known about their trophic behavior in polyculture ponds. The aim of our study was to use carbon and nitrogen stable isotope analysis to characterize trophic niche partitioning between both fish species reared in semi-intensive (fed) and extensive (non-fed) ponds. Fish growth performance was higher in semi-intensive than in extensive ponds. In semi-intensive ponds, δ13C and δ15N values of fish indicated that carp consumed mainly formulated feed, whereas roach also consumed natural food sources. In extensive ponds, δ13C and δ15N values of carp and roach indicated that both fish species did not use the same food sources. Regardless of the type of pond, standard ellipse areas, proxies of the estimated trophic niche size, were significantly smaller for carp than for roach and did not overlap, confirming that roach had more trophic plasticity than carp. Results of this study confirmed that carp and roach are good candidates to be rear in the same pond because they are able to adapt their trophic behavior to reduce trophic competition.