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Leaf carbon isotope ratios were measured on plants from the coastal portions of the Atacama Desert at Pan de Azucar and Paposo, Chile. Most species possessed C-3 photosynthesis, although there were several CAM species, indications of some facultative CAM species, and only one C-4, species. The carbon isotope ratios of the C-3 plants are unusually high, even for species from arid ecosystems. These observations indicated that C-3 species of the Atacama Desert were characterized by very low intercellular CO2 concentration that averaged between 159 and 190 ppm, depending on the severity of the drought.