The responses of three species of nitrogen-fixing trees to CO2 enrichment of the atmosphere were investigated under nutrient-poor conditions. Seedlings of the legume, Robinia pseudoacacia L. and the actinorhizal species, Alnus glutinosa (L.) Gaertn. and Elaeagnus angustifolia L. were grown in an infertile forest soil in controlled-environment chambers with atmospheric CO2 concentrations of 350 μl −1 (ambient) or 700 μl −1. In R. pseudoacacia and A. glutinosa, total nitrogenase (N2 reduction) activity per plant, assayed by the acetylene reduction method, was significantly higher in elevated CO2, because the plants were larger and had more nodule mass than did plants in ambient CO2. The specific nitrogenase activity of the nodules, however, was not consistently or significantly affected by CO2 enrichment. Substantial increases in plant growth occurred with CO2 enrichment despite probable nitrogen and phosphorus deficiencies. These results support the premises that nutrient limitations will not preclude growth responses of woody plants to elevated CO2 and that stimulation of symbiotic activity by CO2 enrichment of the atmosphere could increase nutrient availability in infertile habitats.