Three continuous flow enhanced biological phosphorus removal (EBPR) systems were operated to investigate the effect of the anaerobic SRT on the phosphate removal. The P removal in the system with a short anaerobic SRT declined due to growth of non phosphate accumulating microbes which competed in anaerobic substrate uptake against polyphosphate accumulating bacteria. The phosphorus removal, however, was improved by extending the anaerobic SRT. Restoration and stabilization of P removal by the long anaerobic SRT were confirmed in two other systems.