Abstract: The study on primary productivity of phytoplankton and biomass of plankton ofLongtou reservior was carried out during 1978-1979. The mean primary productivityof phytoplankton is 1.88gC/m² per day. The biomass of phytoplanton and zooplanktonare 5.764mg/L. and 0. 587mg/L respectively. Basing upon the samples of the Longtoureservoir, fish productivity of silver carp (Hypophthalmichthys molitrix) and big-headcarp (Aristichthys nobils) and rational stocking of an aquatic ecosystem are discussed.