The colonization dynamics of ciliated protozoa in a full-scale activated sludge plant and in a RBC pilot plant has been monitored in order to quantify their presence in terms of biomass. In both plants, as the concentration of sludge or biofilm increased, the overall biomass of the ciliate community increased until reaching the highest values during the full-running phase. Ciliate biomass reached values of 250 mg/l (dry weight) in the activated sludge and 314 µg/cm2 (dry weight) in the RBC biofilm. The ciliate biomass/volatile solids ratio was over 9 % for the activated sludge and 12 to 19% for RBC biofilm. High correlation coefficients between ciliate biomass and volatile solids were also observed, which emphasized the close association of ciliate populations with both sludge and biofilm.