Cladoceran assemblage patterns were investigated in a meso-eulrophic reservoir from January 2000 till 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 also assessed. Cladocera and Copepoda dominated the zooplankton community. Cluster analysis performed on the abundance data of higher abundance species (Daphnia longispina, Ceriodaphnia pulchella, Bosmina longirostris and Diaphanosoma brachyurum) helped to identify cladoceran assemblage patterns. ln winter and early spring Daphnia was dominant. ln early summer and end of autumn Ceriodaphnia and Daphnia coexisted. During summer Ceriodaphnia was dominant and coexisted with Diaphanosoma. Bosmina was almost always present, although in low densities. Peaks of abundance of the several populations were not synchronous, evidencing distinct ecological optima. Temperature seemed to be the main structuring factor of this assemblage. The way biotic