Atmospheric CO2 enrichment usually changes the relative contributions of plant species to biomass production of grasslands, but the types of species favored and mechanisms by which change is mediated differ among ecosystems. We measured changes in the contributions of C3 perennial forbs and C4 grasses to aboveground biomass production of tallgrass prairie assemblages grown along a field CO2 gradient (250–500 μmol mol−1) in central Texas USA. Vegetation was grown on three soil types and irrigated each season with water equivalent to the growing season mean of precipitation for the area. We predicted that CO2 enrichment would increase the forb contribution to community production, and favor tall-grasses over mid-grasses by increasing soil water content and reducing the frequency with which soil water fell below a limitation threshold. CO2 enrichment favored forbs over grasses on only one of three soil types, a Mollisol. The grass fraction of production increased dramatically across the CO2 gradient on all soils. Contribution of the tall-grass Sorghastrum nutans to production increased at elevated CO2 on the two most coarse-textured of the soils studied, a clay Mollisol and sandy Alfisol. The CO2-caused increase in Sorghastrum was accompanied by an offsetting decline in production of the mid-grass Bouteloua curtipendula. Increased CO2 favored the tall-grass over mid-grass by increasing soil water content and apparently intensifying competition for light or other resources (Mollisol) or reducing the frequency with which soil water dipped below threshold levels (Alfisol). An increase in CO2 of 250 μmol mol−1 above the pre-industrial level thus led to a shift in the relative production of established species that is similar in magnitude to differences observed between mid-grass and tallgrass prairies along a precipitation gradient in the central USA. By reducing water limitation to plants, atmospheric CO2 enrichment may alter the composition and even structure of grassland vegetation.