We investigated responses of growth, leaf gas exchange, carbon-isotope discrimination, and whole-plant water-use efficiency (W P ) to elevated CO 2 concentration ([CO 2 ]) in seedlings of five leguminous and five nonleguminous tropical tree species. Plants were grown at CO 2 partial pressures of 40 and 70 Pa. As a group, legumes did not differ from nonlegumes in growth response to elevated [CO 2 ]. The mean ratio of final plant dry mass at elevated to ambient [CO 2 ] (M E /M A ) was 1.32 and 1.24 for legumes and nonlegumes, respectively. However, there was large variation in M E /M A among legume species (0.92-2.35), whereas nonlegumes varied much less (1.21-1.29). Variation among legume species in M E /M A was closely correlated with their capacity for nodule formation, as expressed by nodule mass ratio, the dry mass of nodules for a given plant dry mass. W P increased markedly in response to elevated [CO 2 ] in all species. The ratio of intercellular to ambient CO 2 partial pressures during photosynthesis remained approximately constant at ambient and elevated [CO 2 ], as did carbon isotope discrimination, suggesting that W P should increase proportionally for a given increase in atmospheric [CO 2 ]. These results suggest that tree legumes with a strong capacity for nodule formation could have a competitive advantage in tropical forests as atmospheric [CO 2 ] rises and that the water-use efficiency of tropical tree species will increase under elevated [CO 2 ].