Temporal variations in the stable carbon isotope composition (δ13C) of leaves and current-year stems were examined in beech trees over one year. The δ13C of both tissues were equal in the bud stage and started to diverge during growth, with values decreasing by 2·5 and 4·5‰ for stems and leaves, respectively. The dynamics of the δ13C and content of non-structural sugars were also assessed. The beginning of the growth period was characterized by a decrease in starch content and high starch δ13C values. Later in the season, the δ13C of leaf soluble sugars progressively decreased from the end of May and the δ13C of stem sucrose was at least 1·5‰ higher than that of leaves. The δ13C of CO2 respired by stem tissue increased during stem growth and exhibited large seasonal variations ( from −22·1 to −26·3‰). These values generally fell between those of starch and total organic matter. The results of the study showed that the δ13C of stems is altered by two apparent fractionation steps: one during sugar transfer from leaves to stems and one during stem respiration. These results may have implications for analysis of isotopic signals in tree rings and forest ecosystems.