Potential grazing rates on the bloom forming dinoflagellates Prorocentrum minimum and Karlodinium micrum were measured in Chesapeake Bay during summer (2000). Cultured P. minimum and K. micrum cells were fluorescently labeled with 5-chloromethylfluoroscein diacetate and introduced to Ͻ200 m filtered water. Microzooplankton grazing was assessed by measuring the disappearance of labeled prey over time. Grazing on P. minimum and K. micrum was highest between lower oligohaline to midmesohaline regions of the open bay, where microzooplankton biomass was greatest. In June, grazing rates on P. minimum were high at all stations, apparently because of naked (NHD) and thecate heterotrophic dinoflagellates. In July, grazing pressure on P. minimum was related to aloricate oligotrich and choreotrich biomass (r 2 ϭ 0.5107, p ϭ 0.030), whereas g for K. micrum was correlated with Oxyrrhis marina (r 2 ϭ 0.7217, p ϭ 0.004) abundance. In August, grazing on P. minimum was correlated with abundance of the NHD Gyrodinium spp. (r 2 ϭ 0.6621, p ϭ 0.006) and Polykrikos kofoidii (r 2 ϭ 0.6617, p ϭ 0.010) abundance. Microzooplankton biomass peaked within the mesohaline regions of Chesapeake Bay during all months, and these assemblages were dominated by heterotrophic dinoflagellates. On the basis of these results, microzooplankton grazing is an important loss to P. minimum and K. micrum populations in Chesapeake Bay.Photosynthetic dinoflagellates are an important component of plankton assemblages in Chesapeake Bay, forming sporadic blooms throughout the year and contributing substantially to the overall summer primary productivity maximum (Malone et al. 1996; Sellner and Fonda Umani 1999). [ϭGyrodinium galatheanum (Braarud) Taylor], are widespread within the Chesapeake Bay system. Both species form annual blooms, under a broad range of salinity and temperature conditions. Red tides of P. minimum occur on an annual basis in the upper and middle regions of Chesapeake Bay, with cell concentrations sometimes approaching

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