We investigated the grazing impact of dominant zooplankton on Alexandrium sp. populations during the spring of 1998 and 1999, in coastal waters of the Gulf of Maine. At weekly intervals, several stations were sampled for zooplankton and phytoplankton abundance, biomass, species composition, and toxin content. Using natural water samples and wild zooplankton collected from selected stations, we determined feeding rates of zooplankton in natural water samples containing Alexandrium sp. dinoflagellates. Dominant zooplankton included copepods (primarily Acartia hudsonica and Calanus finmarchicus ) and barnacle nauplii (Semibalanus sp.). During 1998, Alexandrium sp. concentrations gradually increased during May, peaking at 3000 cells/L. One week after that peak, Alexandrium sp. concentrations fell to only 30 cells/L. Zooplankton community biomass was low during the initiation of the bloom, but increased exponentially during the period of our investigation. There was also a dramatic change in the zooplankton species composition during the bloom; barnacle nauplii dominated during the early phases of the bloom, and copepods, primarily A. hudsonica, dominated during the later phases. Grazing impacts were low during the bloom initiation and subsequent Alexandrium sp. increase. Concurrent with an increase in the A. hudsonica biomass, grazing impacts increased exponentially (peaking at about 70% / day) and appeared to contribute to the demise of the Alexandrium sp. bloom. In contrast, in 1999, concentrations of Alexandrium spp. remained very low throughout the study period. These findings suggest that grazing can be an important source of mortality and will depend on zooplankton clearance rates, degree of selective feeding, and the biomass and species composition of both the phytoplankton and zooplankton communities.