Recently, some of the variance in primary productivity observed in lakes has been associated with the variability in piscivorous fish populations. This is because various levels of zooplankton consumption by planktivorous fishes result in varying grazing pressures on phytoplankton assemblages. This study proceeds from the idea that in Jordan Lake, zooplanktivory may have strong effects on the composition and chlorophyll concentration of the phytoplankton. The investigation examines the ability of the zooplankton community in a turbid, highly eutrophic southeastern reservoir to control phytoplankton inside enclosures that excluded all fish. The reservoir has a large standing crop of gizzard and threadfin shad, black crappie, bluegill and several other centrarchid and cyprinid planktivores. Six experiments conducted using one meter diameter enclosures between August and September 1986 and May to June 1987 suggested that zooplankton were capable of reducing phytoplankton biomass to very low levels independent of nutrient concentrations when Daphnia spp. was in the lake. The other dominant zooplankton, although increasing in biomass in the absence of fish, did not reduce phytoplankton biomass.