The effects of planktivorous fish on lower trophic levels through predation on zooplankton and nutrient excretion were experimentally separated and their relative importance quantified in a eutrophic humic lake. The experiment was performed in 12 enclosures (3 mi), which initially were identical with respect to all components except fish. At the start of the experiment, caged fish not able to feed on zooplankton were added to four of the enclosures (excretion treatment), and free swimming fish to four enclosures (excretion plus predation treatment). Four enclosures were left as controls. Samples for nutrients and all major groups of organisms from bacteria to zooplankton were taken after 14 and 28 d. The effect sizes of fish excretion and predation were calculated for each variable. Our results suggest that in eutrophic lakes fish predation on zooplankton may be more important than nutrient excretion by fish for the structure and dynamics of planktonic communities. Fish predation on zooplankton was the most important mechanism accounting for fish effects on nutrient concentrations in the water, on phytoplankton biovolume, on rotifers and total zooplankton biomass, as well as on protozoan densities. However our results suggest that nutrient excretion by fish may have important indirect effects on zooplankton. Hence, the effects of plankrivorous fish through both predation on zooplankton and nutrient excretion act in concert acid may be quantitatively important to shape the structure and dynamics of planktonic communities.