Biological phosphorus removal is a biological process responsible for the removal of phosphorus in advanced wastewater treatment plants. Biological phosphorus removal is accomplished by several microorganisms called poly-P bacteria mainly strains of the genus Acinetobacter which have the ability to accumulate phosphorus in excess of their normal cell requirements. The enrichment of an activated sludge plant with a phosphorus removing bacterial population is achieved by sludge recirculation through aerobic and anaerobic zones.Laboratory-scale batch experiments were conducted for a pure culture of Acinetobacter sp. in order to develop a kinetic model that adequately describes the aerobic growth, the behaviour of the bacterium under anaerobic conditions and the transient characteristics of the bacterial cells to changes from anaerobic to aerobic environment and vice-versa. The primary nutrients of interest in the proposed mathematical model were the soluble phosphates and organic carbon and the intracellular polyphosphates and carbon reserves.Experiments were also carried out in a Sequencing Batch Reactor (SBR) using a pure culture of Acinetobacter sp. and synthetic medium in order to secure reproducibility of experimental data. The experimental results showed that phosphorus accumulation in the bacterial cells proved to be dependent on the operational pH.In the sequel, laboratory-scale experiments were conducted in an SBR system using activated sludge as inoculum. The aim of these experiments was to study the behaviour of a mixed culture biological phosphorus removing system.The kinetic model developed for the pure culture οι Acinetobacter sp. was used for predicting the performance of the SBR system under aerobic conditions at different steady states of operation.