The involvement of Acinetobacter in biological excess phosphate removal from the activated sludge process is widely accepted, though its role is not yet clearly defined. To better understand why activated sludge systems remove phosphate, different cell concentrations of Acinetobacter junii were used as inoculum in a mixed liquor medium containing sodium acetate. The phosphate uptake capacity was dependent on the biomass concentration. Low initial biomass concentrations triggered the release of phosphate once transferred into the mixed liquor. Release of phosphate increased during active growth and uptake occurred when cells reached the stationary growth phase. High initial biomass concentration of Acinetobacter junii resulted in uptake of phosphate during the entire duration of the experiment leading eventually to complete phosphate removal.