Abstract: In the period of 1993—1994, we carried out an experiment of the effect of cultivating grass carp in the enclosure on the biodive rsity of zooplankton in Baoan Lake. The paper dealt with reported seasonal varia tion of species composition, density and diversity indexes of zooplankton inside and outside of the enclosure, There were 64 species of zooplankton inside of th e enclosure. and 55 species of zooplankton outside of the outside. The biomass a n d diversity index average annual biomass and of zooplankton were 3738ind/L insid e of enclosure and 3196ind/L outside of enciosure respoctively inside and outsid e of enclosure. The average annual biomass of zooplankton were 08177mg/L insid e and 0.8135mg/L outside inside and outside of enclosure respectively. The averag e annual diversity index of zooplankton were 3.3808 and 2.9717 inside and outsid e enclosure respectively. The result showed that cultivating grazing fishes in the enclosure do not influe nce the standing crop and diversity of zooplankton