Using artifical sewage to culture the single-celled agela (Scenedemus obLiquus Turp) until the biological abandance up to 1.7×10⁴ individual· ml^-1, the river snail (Cipangopaludina sp.) and water fleas (Daphnia obtusa Kurz) were inoculated into the sewage. The population dynamics of the algae and water fleas were described. The results were summarized as follows. (l)The changes of the alga density by times were shown to be exponential function. (2) In the still culture stage without harvest, the population growth curve of Daphnia obtusa Kurz was shown as: InN =-0.06410+0.44041; (3)During the stage of interval harvest, the equation of accummu-lated yield was shown to be diplo-logarithmic function:InN = 0.2836+2.5537 Int. (4)The compositions of nitrogen, phosphorus, carbon of the sewage were analized as the feedback culture decreased 80.60%, 72.40% and 45.70%, respectively.