SUMMARY.The crustacean Zooplankton community in Bough Beech, a flooded valley reservoir in south‐east England, was investigated in the early years of impoundment. The nutrient status and phytoplankton were typical of a lowland eutrophic lake, and this was reflected in the composition of the Zooplankton. At any given time, the latter comprised one or two cladoceran and two or three copepod species, of which the cladocerans were generally numerically predominant. Marked alterations in the Zooplankton community occurred in the first 2 years of the study and were related to changes in predation pressure by fishes. By the end of the third year, the composition of the Zooplankton community had stabilized.The cladoceran biomass was normally in excess of that of the copepods, with maxima in spring and autumn. Peaks in copepod biomass occurred within 2 months of those of the Cladocer