Cladocerans in 19 reservoirs of Guangdong Province, South China were investigated in 2000.Among them, 3 typical with different retention time were sampled in the later 4 years.As a result, 24 species in 16 genera were identified, including 13 eurythermal species, 7 thermophilic species, 3 psychrophilic species and 1 endemic species.There are 5 or 6 species on average in an individual reservoir.The majority of the species were composed of small-bodied species that were abundant and dominant over large-bodied species.These small sized species had a body length less than 1 mm.Small-bodied species such as Bosmina longirostris, Bominopsis deitersi, Diaphanosoma brachyurum, Diaphanosom birger, Ceriodaphnia cornuta and Moina micura were the dominant species.They occurred in all reservoirs less regarding of reservoir trophic states.Among large-bodied species, only Daphnia hyaline and Leptodora kindti can be found frequently in several oligotrophic or mesotrophic reservoirs.The average abundance of cladocerans was very low.In flood season, the average abundance was less than 10 ind./L in 14 investigated reservoirs, and ranged from 10 to 20 ind./L in 5 reservoirs.In dry season, the average abundance was less than lO ind./L in 12 reservoirs, between lO ind./L and 20 ind./L in 5 reservoirs, but reach 67 ind./L in 2 reservoirs:Gaozhou Reservior and Hexi Reservoir.Because of the difference in food quality and quantity of zooplankton, the species composition and population abundance of cladocerans in reservoirs at different trophic states are different to some extent, however, the dominant species were almost the same in these reservoirs.The cladoceran's abundances were related to water retention time in the investigated.The abundance of cladocerans and their food concentration were significantly affected by flushing in the flow-through reservoirs.As most sampling sites are located near dams, the species are mainly contributed by the pelagic group and the composition of species has a typical structure of tropical zooplankton.Cladocerans are dominated by small-sized species;the large sized species have a transparent body that is helpful to survivve in high predation in tropical/subtropical reservoirs.