The effects of cresols on the photosynthesis and growth of natural marine phytoplankton assemblages from the Scripps Pier at La Jolla, CA were measured. In two experiments, in which dinoflagellates were the dominant algae, photosynthesis (measured by 14C uptake) was inhibited at concentrations greater than 100 g/litre and the concentration giving half-maximal photosynthesis was 10000 g/litre. In a third experiment, in which diatoms were dominant, photosynthesis was not inhibited until levels greater than 10000 g/litre were reached and the half-maximal concentration was 20000 g/litre. These experiments suggested that dinoflagellates were more sensitive to cresols than diatoms.