Technical Abstract: While numerous studies have shown that elevated carbon dioxide can delay soil water depletion by causing partial stomatal closure, few studies have compared responses of plant growth to the same soil water deficits imposed at ambient and elevated carbon dioxide. We applied a vacuum to ceramic cups in pots filled with soil to reduce the soil water matric potential to -0.10 MPa. This system was used to maintain a constant mild stress for seven days. In cotton, the stress treatment reduced stomatal conductance at both ambient and elevated carbon dioxide, but the reduction was relatively smaller at the higher concentration. No reduction of photosynthesis measured under the daytime growth conditions occurred at elevated carbon dioxide in stressed cotton plants, while photosynthesis was reduced by the stress in the lower carbon dioxide treatment. The soil water stress treatment reduced the leaf area and biomass of cotton at the lower, but not at the higher carbon dioxide concentration. In soybean, the soil water stress treatment reduced stomatal conductance, photosynthesis and growth at both carbon dioxide levels, but the effect of water stress was not less at elevated than ambient carbon dioxide. In both species, the stomatal closure with the soil water stress may have resulted from the lower soil to leaf hydraulic conductivity. The failure of high carbon dioxide to protect soybean growth from the soil water stress might be related to the lower hydraulic conductivity of stressed soybeans grown at elevated compared with ambient carbon dioxide.