Role of the microorganisms is indispensable for understanding the mechanisum of water purification. We investigated ciliates at Zaru-riber, Umeda-river and Hirose-river in Sendai for 5 months. Values of BOD were about 0.5mg/L at Hirose-river during the period, and were ranged from 0.9 to 2.1mg/L at Zaru-river and Umeda-river. These results showed a good correlation ship between BOD values and the dominant species of microorganisms. Based on these results, we propose a simple example of laboratory work in school. Here, we can say that the diversity of microorganisms supports the potential ability of self-purification in the conservation of river-water.