POTASSIUM AND CESIUM-137 IN BIRGUS LATRO 7 the potassium content determined by flame photometry. Details of the method used have been reported by Chakravarti and Joyner (1959). The ‘levels of cesium-137 were determined by gamma spectroscopy. Five to ten-gram samples of dried muscle were counted. The counting equipmentconsisted of a three-inch thallium-activated sodium iodide crystal used in conjunction with a 256-channel analyzer with digital print out. The total counts per minute under the photopeak were calculated by summing counts per minute of all channels included in the peak and subtracting the background counts. The counting efficiency for the gamma energy measured was determined bycalibrating the instrument with a cesium-137 standard with an error of + 10 percent. The factor used for converting counts per minute to disintegrations per minute was derived from the counting efficiency and from the fractionof the distintegrations giving rise to the gammaradiation being counted. The factor used for cesium-137-barium-137 was 18.0. lies. RESULTS AND DISCUSSION The individual values for potassium and cesium-137 in Birgus muscle collected at Rongelap Atoll and Utirik Island are presented in Table 1. Average values and their standard deviations for each collection also are given. Gor Thelevels of cesium-137 found in individual samples ranged between 731d/m/g dry weight at Kabelle Island, Rongelap Atoll, and 28 d/m/g dry weight at Utirik Island, Utirik Atoll. The individual variation at any one island was large. Range in values in d/m/g/dry weight at three islands at Rongelap Atoll was as follows: Kabelle 324-731, Eniaetok 369-498, and Rongelap 199-330. However, the average ceisum-137 levels were highest at Kabelle Island, which lies in the northern part of the atoll, and lowest at Rongelap Island, in the southern part. Analysis of variance gave an F value of 23.98 (significant at 1 per cent level), indicating that the difference pee nee gar in cesium-137 values between these two islands is statistically significant. At Eniaetok Island, which lies about half way between Rongelap Island and Kabelle Island, only three samples were obtained, two in March 1958, and one in August 1958. In the former collection the levels were higher than at Kabelle while in the latter collection the level was lower than at Kabelle for the corresponding dates. This apparent inconsistency is probably a result of individual variation. At Utirik Atoll, which lies some 200 miles to the east (Fig. 1), the levels were about one-tenth of the lowest levels at Rongelap. (62° ee The gamma dose rates give an indication of the relative levels of radioactive contamination of the different islands. The average gamma dose rates measured three feet above ground in March 1959 at these islands were, Rongelap .031, Eniaetok 051, Kabelle .069 and Utirik .025, expressed as milliroentgens per hour. Determinations of other radionuclides in Birgus muscle indicate that over 90 per cent of the radioactivityis from cesium-137. - For comparison with cesium-137 levels, the average potassium value for each collectionis given in Table 1. The average potassium valueforall samples was 13.05 mg/g with a standard deviation of 3.66. The range in values was 2.96 to 21.2 mg K/g dry weight, but 83 per cent of the 30 samples analyzedfell in the range of 10 to 15 mg K/g dry weight. The two lowest values of 2.96 and 3.48 mg K/g dry weight