Four populations of Sedum wrightii were studied with respect to their photosynthetic carbon metabolism. This leaf—succulent species exhibits crassulacean acid metabolism (CAM). Plants that possess CAM have the potential to vary their rates of transpirational water loss greatly in response to drought. Leaf thickness and biomass °13C values were measured in mid growing season for the populations in their native habitats. The populations exhibited significant differences in both leaf thickness and °13C values ranged from —13.8% to 22.9%. In the field, populations differed in the proportion of day vs. night CO2uptake during growth. Three of the populations were compared in a controlled—environment study. It was found that there are both environmental and genetic determinants of the differences in photosynthetic carbon metabolism. We observed a significant correlation between plant growth and biomass °13C value. The largest plants exhibited the greatest proportion of day vs. night CO2uptake. These finding suggest that in this species there may be an inverse correlation between the ability to converse water and the ability to gain carbon. The unusually wide range of phostosynthetic phenotypes exhibited in this species may well explain its presence over a wide range of environments.