Seasonal changes in species dominance and size structure of littoral Cladocera were investigated in a mainstream reservoir in southwest Virginia. Twenty-four species of benthic, vegetation-dwelling, and pelagic cladocerans were captured during the study. The assemblage was dominated numerically by smallBosminaandChydorus, while larger vegetation-dwelling species such asAlona,Eurycercus,Latona, andSidatypically accounted for the majority of biomass. The number of species captured was relatively constant from May through November with decreasing dominance as the season progressed. Seasonal dynamics of individual species resulted in significant shifts in frequency distributions of numbers and volume across size classes. Overall, the assemblage was divided seasonally as dominance shifted from pelagic to vegetation-dwelling species in mid-July.