Plasticity in water-use efficiency (WUE) was examined in populations of Picea glauca (Moench) Voss and P. sitchensis (Bong.) Carr. and their natural hybrids from an ecocline along the Skeena Valley, British Columbia, which runs from the dry continental interior (P. glauca) to the wet maritime Pacific coast (P. sitchensis). Seedlings were grown in a growth chamber and kept well-watered or repeatedly droughted for 3 months. Mean population photosynthetic WUE and total tissue δ(13)C values were strongly correlated within and across treatments (r=0.95). There were also strong correlations (r=0.60-0.80) between individual seedling total tissue δ(13)C and dry mass, and δ(13)C and net photosynthesis (A), indicating that variation in A was primarily responsible for differences in δ(13)C. When kept well-watered, P. sitchensis and the hybrids had higher δ(13)C (-27.8‰ and -27.5‰, respectively) and higher dry mass (2.17 g and 1.99 g, respectively) than P. glauca (-28.2‰ and 1.68 g). Species ranking was reversed by repeated drought, with P. glauca and the hybrids having higher δ(13)C (-25.6‰ and -25.5‰, respectively) and dry mass (1.10 g and 1.08 g, respectively) than P. sitchensis (-26.4‰ and 0.98 g). P. glauca had a smaller decrease in dry mass (35%) and a bigger increase in δ(13)C (by 2.7‰) than P. sitchensis (55% and 1.4‰, respectively), with the hybrids in between (45% and 2.0‰, respectively). Drought also had a greater effect on A in P. sitchensis (36% reduction) than in P. glauca (14% reduction) or the hybrids (24% reduction). Thus P. glauca and, to a lesser extent, hybrid populations, performed better and were more plastic than P. sitchensis in response to water deficit. Under the well-watered treatment, the hybrids behaved more like P. sitchensis in growth and WUE. These patterns are consistent with the seasonal variation in moisture availability that occurs along the introgression zone.