In view of the high degree of toxicity (Smith 1968, Simpson 1968) of BP 1002, Gamlen Oil Spill Remover, Dasic Slickgone and other "detergents" used in Cornwall to combat pollution from Torrey Canyon oil, it was considered desirable to investigate the toxicity of a compound marketed as an oil dispersant under the brand name "Corexit 7664", claimed by the manufacturers to be non-toxic to marine fauna. It is produced by the Standard Oil Company of New Jersey and marketed in the U.K. by the Esso Petroleum Company. It is stated by the manufacturers to be a non-ionic surfactant, soluble in fresh water, 5% NaCL solution and isopropanol, and dispersible in fuel and crude oils. It contains no organic halides or heavy metals. The investigations reported in this paper were made in two experiments. In the first, the toxic effects of straight dilutions of Corexit in seawater were assessed. In the second, the toxicity of Corexit-dispersed crude oil was compared with that of crude oil alone, with an attempt to imitate conditions at low tide on a polluted beach. The first experiment was carried out in Bantry, Co. Cork, using material collected locally. The second experiment was carried out in the laboratory of the Fisheries Division, Department of Agriculture and Fisheries, and the material was collected at Sandycove, eight miles south of Dublin.