One hundred and sixty-nine bacterial strains were isolated from activated sludge from a waste water treatment basin operating under alternating aerobic/anaerobic conditions. Sixteen strains from a subsample of 23 nitrogen oxide reducers were true respiratory denitrifiers, and all denitrified under both anaerobic and aerobic conditions. REP-PCR band analysis showed different patterns for all strains. One strain (strain 1) produced large amounts of N2O and was studied in detail. Nitrous oxide was the major end product of denitrification by this strain, and NO−2 was reduced more efficiently than NO−3. The aerobic denitrification was most pronounced with NO−2 as electron acceptor, and the reduction of NO−2 was not coupled to NH+4 oxidation.