ABSTRACT Samples of light and dark muscle froin tuna obtained in 1968 and 1969 from the Japanese tuna fishery in the Pacific, and the same tissues plus liver from tuna from the vicinity of Bikini Atoll, were analyzed for gamma-emitting radionuclides, and stable iron. iron-55 Tuna from the southern hemisphere tended to have lower concentrations and specific activities than tuna from the northern hemisphere. There were no significant trends in the 2 5 tes + oa s data when Fe specific activities were compared with species, month of catch, location of catch, or size of fish. Yhere was a . - 55 ties tos . tay a close correlation of Fe specific activity in light muscle, aoa dark muscle and liver muscle end liver. 60 co) : , tained : between dark Yellowfin tuna caught near Bikini Atoll con- . . . Co believed to be derived . close correlation of liver, ~ 35 . and of Fe concentration 60 from the . . atoll. There was a 5 Co concentrations between dark muscle and Cesium-137 concentrations were lower than those reported for fish from middle and northern latitudes by three to five O orders of magnitude.