Effects of anoxic conditions on the growth of nocardioform actinomycetes and of aerobic conditions on growth of Microthrix parvicella in activated sludge were studied to prevent foaming caused by these filamentous microorganisms. The study was conducted on a full-scale plant consisting of three parallel lines fed with the same settled wastewater and subjected to severe foaming due to a massive growth of M. parvicella in winter and of nocardioform actinomycetes in summer The first line was rigged with an initial contact zone (selector) and the data were compared with those observed in one of the lines without a selector: A complete suppression of the growth of nocardioforms was obtained in experiments where returned activated sludge was mixed with wastewater under both anoxic conditions and high sludge loading (11.48 kg BOD5/kg MLSS day). Under moderate sludge loading (1.80 kg BOD5/kg MLSS day) conditions, the selector suppressed the growth of nocardioforms but was not able to improve settlement of the sludge and prevent the growth of other filamentous microorganisms responsible for foaming, such as M. parvicella Finally, a substantial suppression of the growth of M. parvicella was obtained in experiments where returned activated sludge was mixed with wastewater under aerobic conditions and with a high sludge loading (24 kg BOD(5)kg MLSS day); as a consequence, the foaming produced by this filamentous microorganism was highly reduced. (C) 1997 Elsevier Science Ltd.