pilot plant used for the treatment of municipal sewage and pre-treated night soil-containing sewage. In both case, the predominant protozoa was ciliates and among them Vorticella was the most common. In the pilot plant where pre-treated night soil was mixed with municipal sewage, genera of free-swimming ciliates, flagellates and amoeba was higher than in those withour night soil. Correlation analysis on the quality of effluent and protozoa indicates that municipal sewage has positive correlation with protozoa. However in the pilot plan 샐 sewage contatinin pre-treated night coil soil more samples show negative correlation. Followed equations were derived by the regression analysis of BOD in both the pilot plants. In case of pilot plant A of municipal sewage, the analysis B of munici-pal and pre-treated night soil-containing sewage, the analysis of BOD was $6.731$ imes$10_{-2}$</TEX> $ imes$Bodo+0.306(Adjusted $R^2$=0.864). At low temperature, number of protozoa was decreased to 35% and among therm, Aspidisca was the most common genus. Therefore, protozoa can be used as indicator of quality of the effluent in sewage treatment plants