The superiority of large zooplankton in suppressing phytoplankton growth has often been inferred from the Size Efficiency Hypothesis (S.E.H.). The S.E.H. has originally been formulated to account for the competitive superiority of large to small zooplankton under food limiting conditions. Extrapolation of its predictions to the suppression of phytoplankton by zooplankton under high food availability, should be done with care. In an attempt to assess the relevance of the S.E.H. to biomanipulation theory in hypertrophic systems, a fish exclosure experiment was carried out in which the efficiency of two differently structured zooplankton communities in reducing phytoplankton biomass was examined. By inoculating part of the enclosures with laboratory grown Daphnia magna, a community dominated by this large cladoceran species could be compared with a community mainly consisting of Bosmina and smaller Daphnia species. After the exclusion of fish, there was an exponential increase of total. zooplankton biomass. Phytoplankton growth was efficiently suppressed to equal levels in both treatments, though there was a difference in timing: chlorophyll-a levels in the enclosures inoculated with D. magna dropped one week earlier than in non-inoculated enclosures. The time-lag was even more pronounced when large phytoplankton was considered. In accordance with the S.E.H., the time lags could be explained by differences in population growth potential as well as by differences in zooplankton grazing rates (indirectly measured as the minimal zooplankton biomass needed to suppress phytoplankton growth) and food particle size range.