This paper deals w ith the variations of plankton communities in enclosures in which shrimp and tilapia were stocked in f ive densit ies: 2. 8 ind..m- 2, 5. 6 ind..m- 2, 8. 4 ind..m- 2, 11. 2 ind..m- 2, 14. 0 ind..m- 2. The study was conducted in a shrimp farming pond from July to September, 1998 w ith the enclosure - experimental method. The following result s were obtained: ( 1) Because zooplankton feed on phytoplankton, their quantities were in negative correlation in the most enclosures; ( 2) The total biomass of ciliates showed no signif icant correlation with the biomass of zooplankton, but showed negative correlat ion with the biomass of phytoplankton; ( 3) The biomass of phytoplankton was decreased and the saprophilous forms of Pyrrophyta, Crytophyta, Cyanophyta instead of Bacillariophyta and Chrysophyta became dominant group in the enclosures w ith higher shrimp, but zooplankton.. s biomass was increased, supported by the smaller ones; ( 4) Ciliates breed abundantly in higher stocking enclosure, which showed the quality of water becoming bad.