G. galatheanum and P. minimum are two small, bloom-forming dinoflagellates that have been associated with red tides in Chesapeake Bay. Algal blooms are the consequence of net growth, from increase division rates, or decreased mortality rates. In the past, most attention and research has been directed towards variables that increase growth rates, however, little is understood about the impact of predation, or zooplankton grazing, in terms of development and termination of toxic blooms. This study examines the distribution of the potential grazing pressure on G. galatheanum and P. minimum in the Chesapeake Bay and its tributaries. In situ grazing experiments were conducted in June and July, on (2) separate cruises. Dinoflagellate cultures were stained with CMFDA, a vital green fluorscent stain, and added to treatments with (<200 micron) and without (<1.2 micron) the natural microzooplankton assemblages collected at each station. In 58.6% of the experiments, the grazing coefficient was higher the maximum potential growth rate of G. galatheanum and P. minimum in culture, 0.94/d (Li et al., 1999) and 1.15/d (Grzebyk & Berland, 1996), respectively. Thus, grazing by microzooplankton has the potential to prevent net growth of these dinoflagellates. In June, there was a significant correlation between ciliate abundance (>20 micron) and potential grazing on P. minimum. Grazing pressure appeared to be higher in June than in July, but the difference was not statistically significant. In conclusion, microzooplankton have a high potential to control dinoflagellate growth, although the grazing pressure was rather variable throughout Chesapeake Bay and its tributaries.