Wheat, rice and soybean were grown to determine if CO2-induced increases in photosynthesis, biomass, and yield are modified by increases in ultraviolet-B (UV-B) radiation corresponding to stratospheric ozone depletion. The experimental conditions simulated were: (1) an increase in CO2 concentration from 350 to 650 microliters per liter; (2) an increase in UV-B radiation corresponding to a 10% ozone depletion at the equator; and (3) a and b combination. Seed yield and total biomass increased significantly with elevated CO2 in all three species when compared to the control. However, with concurrent increases in UV-B and CO2, no increase in either seed yield (wheat and rice) or total biomass (rice) was observed with respect to the control. In contrast, CO2-induced increases in seed yield and total plant biomass were maintained or increased in soybean within the elevated CO2, UV-B environment. Whole leaf gas exchange indicated a significant increase in photosynthesis, apparent quantum efficiency (AQE) and water-use-efficiency (WUE) with elevated CO2 in all 3 species. Including elevated UV-B radiation with high CO2 eliminated the effect of high CO2 on photosynthesis and WUE in rice and the increase in AQE associated with high CO2 in all species. Elevated CO2 did not change the apparent carboxylation efficiency (ACE) in the three species although the combination of elevated CO2 and UV-B reduced ACE in wheat and rice. The results of the experiment illustrate that increased UV-B radiation may modify CO2-induced increases in biomass, seed yield and photosynthetic parameters and suggest that available data may not adequately characterize the potential effect of future, simultaneous changes in CO2 concentration and UV-B radiation.