Various experimental investigations were carried out in order to develop a novel method of enumerating Nocardia amarae in foaming activated sludge using n-octadecane as a carbon source (OD medium). Colonies of N. amarae isolate appeared about 3 d earlier on OD medium than on MS medium. N. amarae counts on OD medium with 20 mg/l of nalidixic acid, using both synthetic and practical activated sludges, were always higher than those on MS medium. It is concluded that OD medium with nalidixic acid was more effective for the count of N. amarae in activated sludge than MS medium.