Stable isotope analysis is frequently used as a complementary method of dietary analysis, to describe trophic relationships and assess food-web structure. These studies allow a precise determination, based on the calculation of a diet-tissue fractionation factor. The fractionation factor, determined for whole organisms or specific tissues, may vary substantially in natura. In the present study, δ13C and δ15N were assessed in lipid-free tissues (spleen, liver, viscera, scales, gills, spine, white muscle, brain) and in available energy reserves (proteins, glycogen, lipids) of Eurasian perch (Perca fluviatilis) reared under controlled conditions and fed for 4 months with the same artificial diet. Some discrepancies in δ15N and δ13C data were observed among tissues, respectively up to 3.43‰ and 2.54‰ for δ15N and δ13C. The 15N signature in organs depends on their metabolic activity. Despite a significant δ13C enrichment from feed to tissues, the lipids in spine, liver and viscera exhibit a certain stability.