Among 8 strains of typical marine and freshwater photosynthetic bacteria, a Chromatium sp., which is known to flocculate upon the addition of seawater, had the highest intracellular phosphorus content of 0.865 mu mol P/mg dry wt with a ratio of polyphosphate to phosphate (poly-Pi/Pi) of 2.14. Intracellular phosphorus content increased with increasing NaCl concentration up to 260 mM. The poly-Pi/Pi value was also the highest in the medium containing 260 mM NaCl. Sodium cation in seawater enhanced intracellular phosphorus accumulation in this bacterium.