In the cellulose of stems and leaves, δ¹³C was investigated in a birch clone (Betula pendula), which was exposed throughout the growing season to either <3 (control) or 90/40 nl O₃ 1^-1 (day/night). Each regime was split into plants under high or low nutrient supply. δ¹³C was increased (becoming less negative), in stems rather than leaves, by both high nutrition (+2‰) and O₃ stress (+1‰). Whereas high nutrition raised the wateruse efficiency (WUE) while lowering the CO₂ concentration in the inner leaf air space (c _i), WUE decreased and c _i increased under O₃ stress. Therefore, only the nutritional effect on the carbon isotope fractionation was reproduced by the model of Farquhar et al. (1982) which estimates WUE by means of δ¹³C based on c _i. c _i was not biased by ‘patchiness’ in respect to stomatal opening. The latter was verified by microscopical analysis and the complete water infiltration of the birch leaves through the stomata, independent of the diurnal course of the leaf conductance for water vapour. Under low nutrient supply, the activity of phosphoenol pyruvate carboxylase (PEPC) was roughly doubled by ozone to about 1.3% of the total carboxylation capacity (by PEPC + rubisco), and was increased to 1.7% under high nutrition. The fractionation model, extended to account for varying activities of the carboxylating enzymes, indicated that stimulated PEPC was the cause of elevated δ¹³C, although c _i was increased under O₃ stress. The stimulation of PEPC and, as a consequence, elevated δ¹³C are discussed as part of a whole-plant acclimation to O₃ stress.