Plants of Phaseolus vulgaris L. cv. Linden were grown at the current (35 Pa) oř the sub-ambient (20 Pa) partial pressures of CO2 at 29 ± 3 in order to analy2« the photosynthetic acclimatíon to low ambient CO2. No difference was observed in the CO2 response of net photosynthetic rate or leaf conductance (gi) below the CO2 partial pressure measurement of 35 Pa. Above 35 Pa, was depressed in plants grown at 20 Pa CO2 when compared to those grown at 35 Pa CO2. In both treatments, became insensitíve to increasing CO2 above an intercellular partial pressure of 50 Pa, indicating that the capacity of starch and sucrose synthesis to metabolize triose phosphates limited at hi^ measurement CO2. No differences were observed in the ribulose-l,5-bisphosphate carboxylase content, actívity, or actívation statě in plants grown at either CO2 partial pressure. Chlorophyll contents were also equivalent between the treatments. Hence, Uttle modulatíon of enzyme or pigment level follows atmospheric CO2 depletíon, with the possible exception that the capacity of starch and sucrose synthesis may be reduced in plants grown at low CO2.