The stable carbon isotopic composition of organic carbon was measured in 13 species of marine phytoplankton grown axenically in batch culture under identical conditions. The §13 C values ranged from −5.5 to −29 7‰. Two species, Coccolithus pelagicus and Chroomonas sahna, appear able to utilize HCO3- directly. However, the availability of inorganic carbon, as free CO2, accounted for ∼7‰, or 30 per cent of the variance observed It is suggested that differences in isotopic composition between species are primarily due to variations in fi-carboxylation capacities.