To reveal patterns of longitudinal distribution, seasonal variation, and ecological succession in cladocerans communities in the Three Gorges Reservoir, ten sites were used to sample cladoceran communities across a longitudinal gradient from April, 2004 to January, 2005. Nine cladoceran species were collected throughout the year. Species richness was highest in spring (8 species), lower in summer (5 species) and winter (3 species), and lowest in autumn (2 species). Cladoceran species composition varied across sampling sites. Four species were collected from river sampling sites and seven species from reservoir sampling sites. Cladoceran species richness decreased gradually with increasing distance to the dam within the reservoir area. Cladoceran density and biomass varied among seasons (P <0.01) and sampling sites (P <0.01), and both were highest in spring. Density and biomass of cladocerans in river areas were lower than in the reservoir area. Density and biomass of cladoceran in up area of reservoir was lower than that in down area of reservoir. The cladoceran communities of the Three Gorges Reservoir showed distinct patterns in both longitudinal distribution and seasonal variation.