A study was made at nine papermill activated sludge treatment plants, affected by bulking phenomena, on the behaviour of filamentous microorganisms in connection with plant performances. In the sludge samples 24 organisms were identified, the most frequently observed being type 0041 and 0675. The principal causes of inconvenience, isolated through statistical analysis, were identified as low oxygenation, high load, low F/M, nutrient deficiency. Changes made in operating conditions improved bulking in some plants.