Extreme flooding of the Orange River during 1988 led to abnormal dilution of the coastal waters and mass mortalities of intertidal and shallow-water organisms. Species occupying the shallower portions of the subtidal and lower portions of the intertidal zones suffered greatest mortalities. In deeper water mortality was negligible, probably because the fresh water formed a stable layer over the more dense seawater. In the intertidal zone, mortalities tended to be correlated with the physiological tolerances of different species.