Zooplankton in freshwater has been effected by the miniaturization. With the acceleration of this process, the proportion of heterotrophic flagellates in zooplankton has become greater. So, the in situ growth of freshwater heterotrophic flagellates became the important study aspect in ecology in recent years. Moreover, the methodology of in situ study has been renovated. In this paper, the in situ study method was introduced, and results of in situ study were summarized.