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Concentrations of 137Cs in the flesh of albacore tuna caught off the California coast during the last 10 yr decreased by a considerably smaller factor than did corresponding values reported for atmospheric fallout. Between 1965 and 1974, average tissue concentrations decreased steadily from 90 to approx 40 pCi/wet kg, which suggests an effective half time for 137Cs in the upper layer of the eastern North Pacific of about 1 decade. Values for natural 40K, which was measured at the same time, averaged 3300 pCi/wet kg. The 1965 mean concentrations of 137Cs in albacore from four widely separated fisheries in the Northern Hemisphere agreed within a factor of two, ranging from 50 to 90 pCi/wet kg. In contrast, specimens from the South Pacific fishery based at American Samoa averaged only 14 pCi/wet kg. Surface seawater collected around Tutuila Island averaged 0.085 pCi/l. 137Cs, in good agreement with measurements made a year later over a much wider area of the South Pacific. No large variations in flesh concentrations of 137Cs or 40K were observed as a function of tuna size, species, portion of the body sampled or cooking. Thus, radioanalysis of canned tuna may be an efficient method of following major changes in 137Cs contamination of upper layers of the world ocean.