Four populations of Mollugo verticillata L. were compared on the basis of their photosynthetic products, photosynthetic rates, enhancement under low oxygen concentration, and CO2 compensation points. In addition, pulse-chase labeling experiments were conducted using one of the four populations. Depending on the plant population, C4 acids ranged from 40% to 11% of the primary products under short-term exposure to (14)CO2. These compounds were also metabolized during pulse-chase experiments. All four populations had significantly different photosynthetic rates and those rates were correlated with the amounts of labelled C4 acids produced and C4-acid turnover. Three populations of M. verticillata had similar compensation points (40 μl/l) and degrees of photosynthetic enhancement under low [O2] (20%), the fourth population was much lower in both characteristics (CO2 compensation, 25 μl/l; low-O2 enhancement, 12%). The results verify the intermediate nature of photosynthesis in this species, and illustrate populational differences in its photosynthetic and photorespiratory carbon metabolism.