A range of marine phytoplankton was grown in closed systems in order to investigate the kinetics of dissolved inorganic carbon (DIC) use and the influence of the nitrogen source under conditions of constant pH. The kinetics of DIC use could be described by a rectangular hyperbolic curve, yielding estimations of KG(DIG) (the half saturation constant for carbon–specific growth, i.e. Cμ) and μmax (the theoretical maximum Cμ). All species attained a KG(DIC) within the range of 30–750μM DIC. For most species, NH+4 use enabled growth with a lower KG(DIC) and/or, for two species, an increase in μmax. At DIC concentrations of > 1.6 mM, Cμ was > 90% saturated for all species relative to the rate at the natural seawater DIC concentration of 2.0 mM. The results suggest that neither the rate nor the extent of primary productivity will be significantly limited by the DIC in the quasi–steady–state conditions associated with oligotrophic oceans. The method needs to be applied in the conditions associated with dynamic coastal (eutrophic) systems for clarification of a potential DIC rate limitation where cells may grow to higher densities and under variable pH and nitrogen supply.