Fifteen species of planktonic crustaceans, including 7 species of copepod and 8 species of cladoceran, were identified in an investigation for seven reservoirs of Hainan Island in the dry season (December, 2006) and flooding season (May, 2007).All the species were eurythermal or mesophilic, and their communities were dominated by small-sized species.Mesocyclops thermocyclopoides, Bosmina fatali and Bosminopsis deitersi were common species in all the reservoirs.The species richness, in the oligotrophic and oligo-mesotrophic reservoirs were much higher in flooding season than in dry season, but had a reverse pattern in the eutrophic reservoirs.The abundance of Bosmina fatali and Diaphanosoma dubium was positively related with majority of species, and Thermocyclops taihokuensis and Mesocyclops thermocyclopoides were negatively but weakly related with the cladoceran species in correlation analysis.This weak correlations indicated a weak interaction between the species and other influence factors existed, such as fish predation affected coexistence of planktonic zooplankton species.