The relationship between the CO2 accumulating mechanism and carbon isotope discrimination has been investigated in the unicellular green alga Chlorella emersonii. Growth of Chlorella at high C02 levels (5%) which repress the activity of the CO2 accumulating mechanism results in more negative Δδ13C values. The data presented in this paper suggest that it is possible to induce the accumulating mechanism by nitrogen limitation as well as by carbon limitation. Activity of the accumulating mechanism, irrespective of whether it is induced by C or by N limitation, is accompanied by Δδ13C values considerably less negative than those of cells which do not possess such a mechanism. It is suggested that the CO2 accumulating mechanism results in essentially a closed system in which the inherent isotope discrimination by RuBP carboxylase is not expressed.