The capacity for photosynthesis is often affected when plants are grown in air with elevated CO2 partial pressure. We grew Phaseolus vulgaris L. in 35 and 65 Pa CO2 and measured photosynthetic parameters. When assayed at the growth CO2 level, photosynthesis was equal in the two CO2 treatments. The maximum rate of ribulose-1,5-bisphosphate (RuBP) consumption was lower in plants grown at 65 Pa, but the CO2 partial pressure at which the maximum occurred was higher in the high-CO2-grown plants, indicating acclimation to high CO2. The acclimation of RuBP consumption to CO2 involved a reduction of the activity of RuBP carboxylase which resulted from reduced carbamylation, not a loss of protein. The rate of RuBP consumption declined with CO2 when the CO2 partial pressure was above 50Pa in plants grown under both CO2 levels. This was caused by feedback inhibition as judged by a lack of response to removing O2 from the air stream. The rate of photosynthesis at high CO2 was lower in the high-CO2-grown plants and this was correlated with reduced activity of sucrose-phosphate synthase. This is only the second report of O2-insensitive photosynthesis under growth conditions for plants grown in high CO2.