Technical Abstract: The carbon dioxide concentration in free air carbon dioxide enrichment systems (FACE) often has rapid fluctuations. In our FACE system, power spectral analysis of carbon dioxide concentrations measured every second with an open path analyzer indicated peaks in variation centered on a wavelength of one minute. I exposed cotton and wheat plants to either a constant elevated carbon dioxide concentration of 180 ppm above that of outside ambient air, or to the same mean concentration, but with the carbon dioxide enrichment cycling between about 30 and 330 ppm above the concentration of outside ambient air, with a wavelength of one minute. Three replicate plantings of cotton were grown with these treatments for 27 day periods over two summers, and one winter wheat crop was grown from sowing to maturity. In cotton, the fluctuating carbon dioxide treatment consistently resulted in substantial down-regulation of photosynthesis and stomatal conductance. Total shoot biomass of the vegetative plants in the fluctuating carbon dioxide treatment averaged 30% less than in the constantly elevated treatment. In winter wheat, down-regulation of photosynthesis and stomatal conductance also occurred in flag leaves in the fluctuating carbon dioxide treatment, but the effect was not as consistent in other leaves, nor as severe as found in cotton. Wheat yields were 12% less in the fluctuating carbon dioxide treatment compared with the constant elevated carbon dioxide treatment. Comparison with wheat yields in chambers without carbon dioxide addition indicated a non-significant increase of 5% for the fluctuating elevated carbon dioxide treatment, and a significant increase of 19% for the constant elevated treatment. The results suggest that treatments with fluctuating elevated carbon dioxide concentrations could underestimate plant growth at projected future atmospheric carbon dioxide concentrations.