The effect of oxygen on the activity of denitrifying phosphorus removing bacteria has been studied. Denitrifying phosphorus removing bacteria were enriched without oxygen in an anaerobic-anoxic SBR (sequencing batch reactor) over a long time, after which an aerobic phase was introduced into the SBR. The performance on phosphorus and nitrogen removal was examined for an anaerobic-aerobic-anoxic SBR. It could be concluded that oxygen has no detrimental effect on the denitrifying dephosphatation activity. The maximum phosphorus uptake rate by the enriched denitrifying sludge was almost equal for anoxic and aerobic conditions, and the anoxic phosphorus removal activity was kept within at least 5 months after the introduction of the aerobic phase. From the experiments it followed that the advantage of applying denitrifying phosphorus removing bacteria can only be obtained in a pre-denitrification process like the UCT-type of process.