The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO2 concentrations-from sub-ambient to super-ambient-have been studied. Light-saturated rates of net photosynthesis (A (sat)) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 mu L L-1) to current ambient (386 mu L L-1) CO2 growth conditions but did not increase any further as [CO2] became super-ambient (597 mu L L-1). Examination of photosynthetic CO2 response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A (sat) seen was a consequence of growth [CO2] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO2 but could be explained by current models of photosynthesis.