The caesium radionuclides, caesium-137 and caesium-134, are important constituents of aqueous radioactive effluents discharged to the environment of the United Kingdom from fuel re-processing plants and nuclear power stations (Howells, 1966; Mitchell, Harvey & Smith, 1968; Harvey, Baker & Mitchell, 1969). Their accumulation by marine fishes has been the subject of several previous studies, beginning with that of Chipman (1959) who noted that caesium-137 was accumulated in the flesh of the killifish (Fundulus heteroclitus) and that accumulation continued even after a period of 72 days. Similarly, Hiyama & Shimizu (1964) showed that the muscle of the common goby (Acanthobus flaviamus) continued to accumulate caesium-134 from sea water after periods of 30 days. Baptist & Price (1962) have examined the whole body uptake of caesium, by absorption from sea water, in two marine species, the flounder (Paralichthys dentatus) and the Atlantic croaker (Micropogon undulatus), and they also investigated the accumulation, tissue distribution and excretion of caesium-137 in the croaker, the little tuna (Euthynnus alleteratus) and the bluefish (Pomatomus saltatoux), following oral administration of single doses. These authors concluded that the heart, liver and spleen of the croaker absorbed caesium-137 from sea water at a faster rate than the muscle, and that orally administered caesium-137 was rapidly absorbed from the digestive tract. Tissue distributions were similar in the croaker, bluefish and tuna. The retention of caesium-137 in croaker tissue was described as a multiple rate process. In later experiments Hiyama & Shimizu (1969) compared turnover rates in various organs and tissues following uptake from sea water and from single injection experiments, and obtained good agreement between the values from the two methods.