The relationship between carbon isotope discrimination and the specific growth rate of a green alga Chlamydomonas reinhardtii Dangeard (IAM C-238) was studied in the continuous culture system under constant CO₂ pressure. In both the nitrate-limited and the light-limited conditions, carbon isotope discrimination (Δδ¹³C) between alga and CO₂ was dependent upon the specific growth rate of alga. The algal VC value increased with the specific growth rate (μ). In natural environments, the variations in carbon isotope ratio of phytoplankton or particulate organic carbon may be mainly attributed to change in the specific growth rate of phytoplankton, at least for the case in which the δ¹³C values for CO₂ (aq) and HCO_3- in water are kept constant.