The effect of Na+on HCO3-transport, inorganic carbon (Ci) accumulation, and photosynthesis was investigated in the unicellular cyanobacterium Synechocystis sp. PCC6803 using the silicone fluid filtering centrifugation technique. Unlike other cyanobacteria, Synechocystis cells grown at low Ciin standing culture had little capacity for Na+-independent HCO3-transport, when assayed at pH 9.6. However, 25 mM NaCl, but not KCl, strongly promoted HCO3-transport and accumulation. Kinetic analysis indicated that the HCO3-concentration required for one half the maximum rate of transport, K0.5(HCO3-), decreased in the presence of Na+while the maximum rate of transport, VMAX, increased by up to 15-fold. Na+-dependent HCO3-transport occurred against an electrochemical potential of up to 24 kJ ·mol-1, indicating the involvement of carrier-mediated active transport. Li+(1-3 mM) partially substituted for Na+in that K0.5( HCO3-) values were similar (38 vs. 50 µM), but VMAXwas reduced by twofold. At higher concentrations, Li+counteracted the effects of Na+. Monensin reversibly inhibited Na+-dependent HCO3-transport and acted by reducing VMAXwithout affecting K0.5(HCO3-). Monensin inhibition suggested that the electrochemical potential for Na+may play a role in Na+-dependent HCO3-transport, possibly through an involvement in intracellular pH regulation during transport. Na+also stimulated photosynthetic C fixation and O2evolution and these effects were correlated with the Na+-dependent increase in intracellular Ciaccumulation. The Na+-requirement for photosynthesis could be relieved by the provision of CA to the cell suspension, in agreement with the proposal that Na+is required for transport and not directly involved in the photosynthetic process.Key words: active transport, CO2-concentrating mechanism, cyanobacteria, Na+-dependent HCO3-transport, photosynthesis, Synechocystis PCC6803.