Photosynthetic gas exchange, stable carbon isotope discrimination (Delta), height and diameter were compared among five open-pollinated families of 12-year-old western larch trees growing in a common garden in Moscow, Idaho, USA. Statistically significant variation was detected among the families in the two growth traits, Delta and stomatal conductance to water vapor (g) (P </= 0.013), but differences among families in photosynthetic rate (A) and water-use efficiency (W) were not significant (P >/= 0.203). Water-use efficiency was strongly correlated with Delta (r = -0.95, P < 0.01). Neither growth trait was correlated with A (r </= 0.05, P > 0.93) and height was not significantly correlated with Delta (r = -0.75, P = 0.15). Tree diameter and Delta were significantly correlated (r = -0.92, P = 0.03). These results were strongly influenced by a single family. Both the variation in Delta and correlation trends between Delta and the growth traits height and diameter suggest the possibility of selecting for high water-use efficiency with the potential for simultaneous gains in height and diameter growth.