Red water caused primarily by a very high concentration of the dinoflagellateGonyaulax polyǵrammaStein, associated with another dinoflagellate,Prorocentrum micansEhrb., in smaller numbers, appeared in False Bay near Cape Town during March and the early part of April 1962. This bloom resulted in a mass mortality of fish and invertebrates when theGonyaulaxdied in large numbers. The cause of the fauna mortality could not be determined with certainty, but observations indicated that it was due primarily to the depletion of oxygen in the water by the decaying plankton, aggravated by the products of their putrefaction. It was shown that no toxins of the ‘mussel poison’ type were present, but there remains the possibility that a toxin of a type affecting only marine fauna was released. Details of the pattern of mortality are given and the hydrological and meteorological conditions associated with this phenomenon are described. It is suggested on the basis of published reports that mussel poisoning and mass mortalities of marine fauna are distinct phenomena caused by different species of dinoflagellates.