Five sewage-treatment plants were studied for 1 year including the two plants of the Beijing Gaobeidian Sewage Treatment Factory, the largest activated-sludge scheme in China, with a treatment capacity of one million tons of sewage daily. Vorticella convallaria, Epistylis plicatilis, Aspidisca sulcata and Arcella hemisphaerica occurred frequently at all five plants and usually appeared in large numbers in the mixed liquor. Rotifers occurred frequently and in moderate numbers at all five plants. The two plants at Gaobeidian had a high degree of similarity, and the fauna structure of Beixiaohe Plant was similar to that of Fangzhuang. Rotifers showed a negative association with the reduction of BOD 5 , COD and suspended solids removal at two plants. The appearance of many Arcella hemisphaerica was associated with good settlement at two plants.