Elevated atmospheric CO.sub.2 concentration increases the performance of invasive plants relative to natives when grown in monoculture, but it is unclear how that will affect the relative competitive abilities per se of invasive and native grasses grown together. We tested competitive outcomes for four native and four invasive perennial C3 and C4 grasses under ambient (390 ppm) and elevated (700 or 1000 ppm) CO.sub.2 concentrations in the greenhouse with non-limiting water and nutrients. We predicted that elevated CO.sub.2 would increase the competitive suppression of native grasses by invasive grasses. To test this, we determined the relative interaction intensity of biomass allocation for natives grown alone vs. those grown in native-invasive species pairs. We also measured photosynthetic traits that contribute to plant invasiveness and may be affected by elevated CO.sub.2 concentrations for species pairs in mixture to determine native-invasive relative performance. We found no effect of CO.sub.2 for the aboveground biomass and tiller production measures of interaction intensity or for relative performance for most of the measured photosynthetic traits. In competition, the invaders nearly always outperform natives in biomass and tiller production, regardless of CO.sub.2 level. The results suggest that increasing CO.sub.2 concentration alone has little effect on grass competitive outcomes under controlled conditions.