The development of an inorganic carbon concentrating mechanism (CCM) in relation to the expression of extracellular carbonic anhydrase (CAext) in cells from the stationary phase of Emiliania huxleyi cultures was investigated. Unlike exponential phase cells, those in the stationary phase showed a high affinity for CO2: the K0.5[CO2] (i.e. the CO2 concentration required for half-maximal rate of photosynthesis) was 1.6μM at pH 8.3. Measurement of the internal inorganic carbon concentration by the silicone oil centrifugation technique showed that, with 1 mM external dissolved inorganic carbon (DIC), the intracellular inorganic carbon concentration was sixfold greater at pH 7.5 and twofold greater at pH 8.3, than the external concentration. The potent CAext inhibitor, dextran-bound sulphonamide (DBS), gave an 80% inhibition of DIC-dependent photosynthetic oxygen evolution. Similarly, DBS decreased the intracellular DIC concentration and inhibited photosynthetic 14CO2 fixation. The lipid-permeable CA inhibitor, ethoxyzolamide (EZ), increased the intracellular DIC concentration but inhibited photosynthetic 14CO2 fixation. The observed intracellular DIC concentrations indicated that the stationary phase cells have the capacity to transport CO2 actively against a concentration gradient.