Abstract: The water system in the Yanshan region, Beijing, including Zhoukoudian River (St. 16, 18), Dongsha River (St. 7, 10, 12, 14), Dingjiawa River (St. 20, 23), Xisha River (St. 24, 25) and the middle reaches of Dashi River (St. 1, 3), was polluted by industrial Wastewater and municipal sewage from the Yanshan Petrochemical Company (Fig. 1, Tab. 1, 2). The most harmful contaminents were oil, phenol, and strongly acid or alkaline effluents. The present study aims at an evaluation on the ecological effects of these pollutants based on the protozoan communities. From April to June of 1985, protozoan communities were collected from the mentioned five rivers simultaneously with three methods: (1) slide method for periphytic protozoans, (2) PFU method for protozoan colonization, (3) classical method of planktonic settling. The upstream Dashi River (St. 1) was used as the control station for the assessment of water quality. 351 protozoan species (84 for Phytomastigophora, 34 for Zoomastigophora, 54 for Sarcodina, 179 for Ciliata) were found in the rivers. Compared with the result of the control station, the number of protozoan species in Zhoukoudian and Dongsha rivers decreased dramatically (Tab. 3). It was found that all of the three methods are effective for monitoring the deterioration of water quality in the upper reaches of the Zhoukoudian River (St. 16), where protozoans were practically wiped out due to the sharp and alternative change of acid and alkaline effluents (Tab. 4—7, Fig. 2). In Dongsha River, polluted by the petroleum refinery and municipal effluents, the results of slide method and PFU method both indicated a gradual selfpurification process. The species diversity index of this river increased gradually toward downstream reaches (St. 12, 14) where water quality became recovered and the attached ciliates (Peritrichida) were predominant in station 10 (Tab. 4). The slide method was incapable of evaluating the difference of pollution between Xisha River and Dingjiawa River. The PFU method has the ability of examining the structural and functional processes at community level, such as species diversity, cluster analysis of similarity coefficients (Fig. 4), and colonization (Fig. 2, 3, Tab. 5). Based on the structural and functional parameters, the PFU method could differentiate the nature of Xisha and Dingjiawa rivers. To compare the quality of the rivers, results indicate that the toxic stress increased in sequence as Dashi River→Dingjiawa River→Xisha River→Dongsha River→Zhoukoudian River. Because of receiving waste discharges from the other four rivers, the middle reaches of Dashi River (St. 3) exhibited eutrophic phenomena, such as the presence of heterotrophs and lower indices of species diversity, as compared with the producer-dominated and higher diversity indices in the upper reaches of that river (St. 1). Therefore, the potential pollution of Dashi River is threatening the North Juma River, one of the important water resources of Hebei Province. Pollution control of the water system in Yanshan region is becoming an urgent problem.