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Plant growth, biomass allocation, carbon isotope composition (delta C-13), and water use efficiency (WUE) of 31 cultivars of apple (Malus domestica Borkh.) grown under two water regimes were measured. Drought-stressed plants showed significant declines in tree height, trunk diameter, biomass production, and total leaf area, the extent to which depended upon cultivar. Also, gas exchange rates, instantaneous and long-term efficiencies (WUEI and WUEL, respectively), and values for delta C-13 differed among cultivars and watering regimes. Variations in WUEI were mainly due to changes in stomatal conductance (g (s)) under drought condition. ’Qinguan’ and ’Golden Delicious’ had greater trunk diameter, tree height, and had higher biomass production and WUEL under drought stress, implying that they are more suitable for arid and semi-arid regions. Moreover, WUEL was significantly and positively correlated with delta C-13 under two watering regimes, which suggests a potential for evaluating water use efficiency of Malus by measuring carbon isotope composition.