(13)C/(12)C ratios have been found in totally combusted leaves of Crassulacean acid metabolism plants to range from -14 to -33 delta (13)C per thousand compared with a limestone standard. Crassulacean acid metabolism plants apparently utilize both ribulose-1, 5-diphosphate carboxylase and phosphoenolpyruvate carboxylase to assimilate atmospheric CO(2) and, depending on environmental conditions, have (13)C/(12)C ratios indicative of either carboxylase or to any intermediate value. The degree of discrimination against (13)C and the resultant (13)C/(12)C ratio from the photosynthetically fixed CO(2) is influenced by environmental conditions and is not a specific and fixed characteristic of a Crassulacean acid metabolism plant. Certain Crassulacean acid metabolism plants may shift their ratios as much as 17 delta (13)C per thousand in specific environments.