The bioluminescent, photosynthetic, tropical marine dinoflagellate Pyrodinium bahamense occurs in persistent high concentrations in Oyster Bay, Jamaica, W. I., although mean tidal flushing rates determined by dye‐tracer techniques would predict their disappearance. As a result of a unique temporal sequence of positive phototaxis and the differential movement of diurnal wind‐driven water layers of different densities, there is a daily transfer of P. bahamense into the eastern shallows of the bay where flushing rates are minimal, reducing the effect of the mean bay dilution. A second phototaxis‐dependent mechanism in the eastern shallow results in zones of high concentration containing up to 10,000,000 P. bahamense per liter. This latter mechanism can explain the production of red water outbreaks in coastal waters. Nitrogen and phosphorus levels observed are consistent with both mechanisms.