Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33Β°30' to 53Β°12' north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination (Ξ) ranged from 19.70(β°) to 22.43(β°) and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE=0.21)β°) was not significantly different from the interior variety (20.91 (0.15)β°). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)β°) (lowest water-use efficiency). Carbon isotope discrimination (Ξ), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO concentration (c/c), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r=0.76, 0.73, 0.74, and -0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at Pβ€0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and Ξ, in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and Ξ with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteri tics in situ.