Abstract: In this paper, the quantitative dynamics of two heterotrophic flagellat populations, M. socialis and B. saltans which were separated from a PFU sample of Lake Donghu and cultured in Ceraphyl media of various concentration, is studied comparatively. The result shows that the quantitative dynamics of each heterotrophic flagellate population differs significantly from the other. The population growth curves of M. socialis coincide with the logistic curve, but the population growth curves of B. saltans are parabolic. The population growth equations for the two species are established.