Long-term effects of At ion addn. 2.0-2.5 g/kg.d related to the dry mass of activated sludge on the growth of filamentous bacteria (Microthrix parvicella, Nostocoida limicola, 021N) in bioreactors of the municipal wastewater treatment plant in Gdansk were studied in 2000-2007. Com. poly At chloride was used as the source of At ions. The addn. resulted in disappearing the biol. foam both in the bioreactors and clarifiers and in a redn. of the activated sludge swelling index from 260 down to 135-155 mL/g of the dry biomass. After the expts. were completed, the At doses were reduced down to 1.4-2.2 g/kg.d.