The interrelationship of zooplankton and bacteria in the limnic food chain has become of increasing interest. Numerous authors have investigated the grazing of different groups of zooplankton on freshwater bacteria: ciliates (Taylor and Berger 1976, Taylor 1978,1979), rotifers (Dougherty et al. 1960, Pilarska 1977, Starkweather et al. 1979, Bogdan and Gilbert 1982, Pourriot 1980, Starkweather ... [Show full abstract] 1980), cladocerans (McMahon and Rigler 1963,1965, Fenchel 1980,Haney 1973,Gophenetal. 1974,Lampert 1974, Kerfoot and DeMott 1980, Hadas et al. 1982, 1983, Pace et al. 1983), and copepods (Malovitskaya and Sorokin 1961, Rieper 1978). The predatory pressure on natural assemblages of bacteria caused by zooplankton communities has also been investigated (Gliwicz 1969, Haney 1973, Crisman et al. 1981, Pace and Orcutt 1981, Pedrós-Alió and Brock 1983). Of special interest is the impact of microzooplankton (heterotrophic nanoflagellates, protozoa) grazing on bacteria (Sherr et al. 1987), which is described in Chapter 12 (this volume).