Protozoa are an important component of both the nano- and microplankton in marine and freshwater environments and are preyed upon by zooplankton, including suspension-feeding cope pods, some gelatinous zoopiankters and some first-feeding fish larvae. The clearance rates of suspension-feeding zooplankton for ciliates, in particular, are higher than for most phytoplankton. For at least some suspension-feeding zooplankton, protozoans are calculated to be quantitatively an important component of the diet during certain seasons. In laboratory studies, protozoan components in the diet appear to enhance growth and survival of certain life-history stages or enhance fecundity. These data suggest that protozoans are qualitatively as well as quantitatively important in the diets of marine zooplankton. Most studies of predation on Protozoa have focused on the euphotic zone in nearshore waters. Predation on Protozoa is expected, however, to be particularly important both quantitatively and qualitatively in marine environments and seasons in which primary production is dominated by cells <5 μm in size, such as nearshore environments after the spring phytoplankton bloom, in oligotrophic waters, and in environments dominated by detritus-dominated food webs, such as the deep sea. In detritus-dominated food webs, Protozoa may be a source of essential nutrients and may thus facilitate utilization of bacterial and detrital carbon by metazoan plankton.