Many operational problems associated with abnormal foaming or scum formation in activated sludge processes have been reported to be caused by accumulation of actinomycetes, i.e. Nocardia amarae and Rhodococcus spp, A one-year surrey of N. amarae profiles in an actual foaming activated sludge plant was carried out based on viable cell count measurement with use of OD medium, containing n-octadecane as a carbon source. The amount of N. amarae in the mixed liquor increased and that in the suspended solids remained almost constant during the steps of conversion of the raw sewage to the return sludge, but that in the scum was large, Therefore, sludge retention time (SRT) and sludge age (SA) of N. amarae were longer than those of activated sludge microorganisms, In analysis of mass balances of N. amarae, it was found that about 95% of the N. amarae in the secondary clarifier influent was returned to the aeration tank and that N. amarae could grow in the aeration tank, Based on the findings, countermeasures to prevent scum problems in this plant were proposed and discussed from a practical point of view.