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High abundances of dinoflagellates in mixed phytoplankton populations in marine enclosures were strongly correlated with high pH during 23 enclosure-years of weekly samples. Diatom blooms were not similarly correlated with high pH. The correlation with high pH was not the result of dinoflagellate blooms themselves drawing down the CO2 and driving up the seawater pH. Examination of individual blooms of >500 cells ml-1 indicates that dinoflagellate cell counts increased only after the pH was driven high (i.e. >8.5). High pH occurred either by natural processes (diatom blooms) or, in one case, by an artificial manipulations of the pH in the enclosure. There were 9 periods in which the seawater pH exceeded 8.5. Dinoflagellate blooms occurred during 7 of those events. A high pH affinity for dinoflagellates could help explain reported successional sequences of diatom blooms followed by dinoflagellate blooms and the association of dinoflagellate blooms with eutrophication. Seawater pH should probably be included with other environmental factors in studies of the mechanisms that control the occurrence of field dinoflagellate blooms.