The trophic state and plankton distribution were investigated in Hedi reservoir in 2000.The TLI(trophic level index) showed that the reservoir was meso-eutrophic.The abundance of phytoptankton in flood season and dry season were 1 950.5～3 509.7×10⁴ cells·L^-1and 130.5～231.3×10⁴ cells·L^-1respectively,and the proportion of Microcystis sp.and Phormidium sp.(Cyanophyta) in phytoplankton abundance were over 83%.In both flood season and dry season,the phytoplankton community structure is Cyanophyta-Chlorophyta-Bacillariophyta.The dominant species are Microcystis sp.and Phormidium sp.in the two seasons.The sub-dominant species were Gloeocapsa sp.and Oscillatoria sp.(Cyanophyta),Chlamydomonas sp.(Chlorophyta),and Melosira granulata(Bacillariophyta)in the flood season.However,the sub-dominant species were Aphanizomenon flos-aquae(Cyanophyta) and Melosira granulata(Bacillariophyta)in dry season.All above these species are eutrophic indicators.The species of zooplankton in the Hedi Reservoir were rare but the abundance very high.In the flood season,the Nauplii and Copepodite of Copepoda were abundant in the reservoir.The predominated species were Keratella valga,Trichocerca elongata,Brachionus forficula(Rotifer)and Bosmina longirostris(Cladocera)in the flood season.In the dry season,however,the abundance of Nauplii were highest in zooplankton but the abundance of Copepodite descended rapidly.The predominated species were Bosmina longirostris(Cladocera),Mesocyclops leuckarti(Copepoda)in dry season.The predominated groups in the two seasons were those species with a wide-ecological spectrum and/or were all organic matter-resistant.