A new ammonia-oxidizing strain, isolated from an aerobic biofilm in a domestic sewage-treatment system, was identified as a species of Nitrosomonas different from Nitrosomonas europaea. This strain had morphological features and growth characteristics typical of members of the genus Nitrosomonas. The G+C content of the DNA of this strain was 48.5 mol%, being lower than that of known strains of N. europaea. The extent of the homology between the DNA of this strain and that of other strains of N. europaea was less than 30%. After cells of this isolate, immobilized in a polyacrylamide gel, had been added to the aerobic reactor of a laboratory-scale sewage-treatment system, the concentration of ammonium nitrogen in the effluent decreased to 2 mg/l without the accumulation of nitrite, and removal of more than 70% of the nitrogen from the input sewage was achieved.