Stable carbon isotope composition was determined on leaves of woody plants sampled along an 800-km transect on the western flank of the Rocky Mountains at altitudes ranging from 610 to 2650 m above mean sea level. Discrimination decreased by 1.20 +/- 0.11 parts per thousand (mean +/- 1 SE) per km of altitude (n = 15, F-1,F-13 = 127.8, P < 0.0001). The change in discrimination was just sufficient to maintain a constant CO2 partial pressure gradient from ambient air to the intercellular spaces within the leaf for both deciduous (P = 0.60) and evergreen (P = 0.90) species. However, the CO2 gradient so maintained was significantly steeper among evergreen (11.31 +/- 0.14 Pa) than among deciduous (9.64 +/- 0.14 Pa) species (t = 8.4, 27 df, P < 0.0001). As a consequence, the evergreens had lower discrimination than the deciduous species at any given altitude. After the data were corrected for altitude, further analysis revealed significant differences in discrimination and in CO2 partial pressure gradient among species. Thuja plicata (western red-cedar), a scale-leaved evergreen, had lowest mean discrimination (16.67 +/- 0.50 parts per thousand, n = 4) and the steepest CO2 gradient from ambient to intercellular spaces (14.5 +/- 0.5 Pa). Larix occidentalis (western larch), a deciduous conifer, had the highest discrimination (20.95 t 0.34 parts per thousand, n = 9) and the flattest CO2 gradient (8.3 +/- 0.4 Pa), A simple model of water-use efficiency predicted that evergreen species would average 18 +/- 2% higher in water-use efficiency at any given altitude and that mean water-use efficiency would triple across a 2000-m altitude gradient. The difference between evergreen and deciduous species is attributable to variation in the CO2 partial pressure gradient, but the tripling with altitude was almost exclusively a consequence of reduced evaporative demand.