Differential responses of species to environmental stress may interfere with restoration of prairie ecosystems or change community structure. The impact of increasing atmospheric ozone (O3) concentrations and/or low water on the growth of Andropogon gerardii Vitm. (big bluestem) and Sorghastrum nutans (L.) Nash (indian grass), two common warm-season native grasses, and Setaria faberi Herrm. (giant foxtail), a vigorous annual weed species, were studied in replacement series. Giant foxtail grew better than either big bluestem or indian grass under all tested conditions. The leaf areas of all three species were primarily controlled by water availability. Big bluestem and indian grass accumulated biomass equally well under high water availability, but with low water, indian grass accumulated more biomass than did bluestem. Three-way analysis of variance showed biomass, leaf area, and leaf number differed among species; low water was significant in all cases except for indian grass leaf area; and the O3 effect was significant only in the case of foxtail biomass. The interaction of O3 concentration and low water was significant only for indian grass biomass and leaf number; the interaction of species combination and low water was significant only for big bluestem leaf area and biomass. Relative yield calculations indicated that under conditions of elevated O3 and low water, big bluestem was the least competitive, while indian grass was most competitive. Intraspecific competition was common, each species apparently utilizing the environment in different ways. The results also suggest that giant foxtail at a low relative density may be used as a nurse species in prairie restorations as growth of big bluestem and indian grass were improved when in mixtures with foxtail.