Laboratory experiments were done to measure the potential grazing pressure of < 200 µm fraction of plankton on Pfiesteria piscicidia and to determine which taxa are important predators on this dinoflagellate. During the summer of 1999, water samples were collected from tidal estuaries on the eastern shore of the Chesapeake Bay from which P. piscicida has been reported. The water was gently filtered through a 200 µm mesh to remove macrozooplankton. The control treatment was filtered thru a GF/C filter to remove nano- and microzooplankton as well. Non-toxic zoospores (NTZ) of P. piscicida (Strain FDEPMDR23, an apparently non-toxic strain) were stained with a vital green fluorescent dye, 5-chloromethylfluorescein diacetate, and added at concentrations of 500 to 1000 cells/ml to the treatments. Number of ingested zoospores per grazer was determined after 10-15 mins. Number of zoospores was enumerated hourly. The dominant micrograzers on P. piscicida were certain species of large tintinnid and oligotrich ciliates. Metazoa and non-ciliate protistan taxa were usually minor contributors to grazing by the <200 µm fraction on P. piscicida. The instantaneous rate of grazing mortality varied from 0 to ˆ0.46/h (instantaneous rate of growth of NTZ‚s varies from ~0 to 0.08/h, depending on irradiance and prey concentration). These results indicate the protistan grazing may, at times, regulate or prevent proliferation of NTZs, but that grazing mortality is highly dependant on species composition as well as abundance of ciliate microzooplankton. Times when grazing pressure are low, in combination with high algal prey densities, may present windows of opportunity for growth of Pfiesteria NTZs.