Dinoflagellates were counted and identified in >20 v samples from Buzzards Bay, Massachusetts, USA, from October 1987 to September 1988. Eight stations were sampled at monthly or twice-monthly intervals. Dinoflagellates were the most abundant organisms in the >20 pm microplankton samples (excluding diatoms), comprising up to 99% of total organisms. A total of 46 dinoflagellate species from 19 genera were recorded, and there were dinoflagellate blooms throughout the year. Bloom-forming taxa included Gymnodinium sanguineurn, Heterocapsa triquetra, Prorocentrum minimum, and Prorocentrum triestinum. Mean annual dinoflagellate abundance was 9.7 X 103 cells I-', but blooms reached concentrations of 10' cells 1-'. Dinoflagellates were most abundant from spring to fall, and dinoflagellate blooms developed in between diatom blooms, coinciding with increasing silicate concentrations. This suggests that after silicate depletion by diatom blooms, dinoflagellates were able to exploit an undersaturated phytoplankton niche. Dinoflagellates were much more abundant at the New Bedford sewage outfall and in New Bedford Harbor than in the rest of the bay. The effect of concentrating dinoflagellate samples with a 20 pm mesh screen was examined by comparisons of abundance of selected taxa in screened and unscreened whole-water samples concentrated by sedimentation. There were no significant differences between screened and whole-water samples (2-way ANOVA for paired comparisons) for total dinoflagellates, or for 5 abundant taxa.