Immobilization of Acinetobacter cells can provide a method which could possibly be used to explain the mechanism of biological phosphorus removal from AS systems since this may allow the study of pure cultures in their natural habitat. Pure cultures of Acinetobacter johnsonii and Acinetobacter calcoaceticus cells were therefore immobilized within alginate beads (3% and 3.5% alginate) to assess their behaviour and particularly, their survival, leakage, growth and phosphate uptake abilities. Both species survived immobilization conditions, however, the effects on viability, leakage rate, growth and phosphate uptake were dependent upon the alginate concentration and the strain. Notwithstanding leakage, cell densities within pure cultures over 24 h were higher than initial densities due to growth. The growth rate of A. calcoaceticus immobilized cells was twice that of free cells, while A. johnsonii showed similar growth rates between immobilized and free cells. Non growing immobilized cells showed higher phosphate uptake ability than growing cells. After 24 h, A. calcoaceticus immobilized cells removed more phosphate per cell than A. johnsonii cells while both species showed higher phosphate uptake ability when immobilized in 3% alginate than in 3.5% alginate.