Dioecious species represent an important componenet of terrestrial ecosystems, but little is know about gender-specific responses to elevated atmospheric CO2. In an open-top chamber experiment carried out in Michigan, USA, the physiological and growth responses were studied of male and female Populus tremuloides to elevated CO2 and soil nitrogen concentrations. Male trees had a higher net photosynthetic rate than female trees, but the difference was greater at elevated (25%) than at ambient (13%) CO2. Leaf dark respiration, averaged across the growing season, tended to be higher in males than in females, and increased significantly in male and female trees with CO2 enrichment. Female trees had higher total biomass than male trees grown in low-nitrogen soil and at ambient CO2, but not in other treatments. Elevated CO2 increased the total biomass of males by 58-66% and of females by 22-70%. Differing physiological and growth responses to CO2 enrichment by male and female trees should be taken into consideration when predicting the effects of global environmental changes on forest ecosystem structure and functioning.