Some physiological characteristics of photosynthetic inorganic carbon uptake have been examined in the marine diatoms Phaeodactylum tricornutum and Cyclotella sp. Both species demonstrated a high affinity for inorganic carbon in photosynthesis at pH7.5, having K1/2(CO2) in the range 1.0 to 4.0mmol m−3 and O2− and temperature-insensitive CO2 compensation concentrations in the range 10.8 to 17.6 cm3 m−3. Intracellular accumulation of inorganic carbon was found to occur in the light; at an external pH of 7.5 the concentration in P. tricornutum was twice, and that in Cyclotella 3.5 times, the concentration in the suspending medium. Carbonic anhydrase (CA) was detected in intact Cyclotella cells but not in P. tricornutum, although internal CA was detected in both species. The rates of photosynthesis at pH 8.0 of P. tricornutum cells and Cyclotella cells treated with 0.1 mol m−3 acetazolamide, a CA inhibitor, were 1.5- to 5-fold the rate of CO2 supply, indicating that both species have the capacity to take up HCO3− as a source of substrate for photosynthesis. No Na+ dependence for HCO3− could be detected in either species. These results indicate that these two marine diatoms have the capacity to accumulate inorganic carbon in the light as a consequence, in part, of the active uptake of bicarbonate.