The marine photosynthetic bacterium Chromatium sp, successfully removed orthophosphate when grown phototrophically. The phosphate-uptake rate was almost constant at more than 5.0 mg- PO43-/l in synthetic medium. Addition of seawater causes flocculation of this strain. The successful use of seawater as an inexpensive source of magnesium could prove to be effective in the removal of photosynthetic bacterial cells from a medium. A semicontinuous culture system was used for the removal of low concentrations of phosphate and the phosphate-uptake activity of Chromatium sp. was maintained under 0.1 day(-1) dilution rate. This strain was also able to remove high concentrations of phosphate from domestic sewage.