In this work the quantitative importance of ciliated protozoa in the activated sludge and biofilm was determined in terms of biomass. The colonisation dynamics of ciliates in a full-scale activated-sludge plant and in a rotating biological contactor (RBC) pilot-plant were monitored. In both plants, as the concentration of sludge or biofilm increased the overall biomass of the ciliate community increased and reached the highest values during the steady-state phase. Ciliate biomass reached values of 250 mg/litre (dry weight) in the activated sludge and 314 mug/cm2 (dry weight) in the RBC biofilm. The ciliate biomass was over 9% of the volatile solids for the activated sludge and 12-19% for the RBC biofilm. Large correlation coefficients between ciliate biomass and volatile solids were also observed, which emphasised the close association of ciliate populations (mainly crawling and attached forms) with both sludge and biofilm.