Polyphosphate (polyP) is a ubiquitous biopolymer whose function and metabolism are incompletely understood. The polyphosphate kinase (PPK) of Acinetobacter sp. strain ADP1, an organism that accumulates large amounts of polyP, was purified to homogeneity and characterized. This enzyme, which adds the terminal phosphate from ATP to a growing chain of polyP, is a 79-kDa monomer. PPK is sensitive to magnesium concentrations, and optimum activity occurs in the presence of 3 mM MgCl₂. The optimum pH was between pH 7 and 8, and significant reductions in activity occurred at lower pH values. The greatest activity occurred at 40°C. The half-saturation ATP concentration for PPK was 1 mM, and the maximum PPK activity was 28 nmol of polyP monomers per µg of protein per min. PPK was the primary, although not the sole, enzyme responsible for the production of polyP in Acinetobacter sp. strain ADP1. Under low-phosphate (P_i) conditions, despite strong induction of the ppk gene, there was a decline in net polyP synthesis activity and there were near-zero levels of polyP in Acinetobacter sp. strain ADP1. Once excess phosphate was added to the P_i-starved culture, both the polyP synthesis activity and the levels of polyP rose sharply. Increases in polyP-degrading activity, which appeared to be mainly due to a polyphosphatase and not to PPK working in reverse, were detected in cultures grown under low-P_i conditions. This activity declined when phosphate was added.