The assemblage patterns of pelagic Cladocera species were investigated in a meso-eutrophic reservoir from January 2000 to December 2002. Trends of environmental factors (water temperature, conductivity, pH, and nutrient concentrations), as well as of biotic factors (availability of potentially edible phytoplankton and densities of herbivorous and carnivorous copepods) were assessed. Zooplankton community was dominated by Cladocera and Copepoda. Assemblage patterns for Cladocera were identified by performing a cluster analysis on abundance data of the dominant species: Daphnia longispina, Ceriodaphnia pulchella, Bosmina longirostris and Diaphanosoma brachyurum. In winter and spring, Daphnia was dominant. In early summer and end of autumn, Ceriodaphnia and Daphnia coexisted. During summer, Ceriodaphnia was dominant and coexisted with Diaphanosoma. Bosmina was present in low densities during the whole study period. Cladoceran populations exhibited non-synchronous peaks of abundance, evidencing different ecological optima. Temperature seemed to be the main structuring factor of this assemblage. The way biotic interactions influenced pelagic cladoceran assemblage in this reservoir was not clear and needs further research.