equipment consisted of a three-inch thallium-activated scdium iodide crystal used in conjunction with a 256-channel analyzer with digital print out. The total counts per minute under the photopeak were cal- culated 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 by calibrating the instrument with a cesium-137 standard with an error of +t 10 per cent. The factor used for converting counts per minute to disintegrations per minute was derived from the counting efficiency and from the fraction of the disintegrations giving rise to the gamma radiation being counted. The factor used for cesium-137 - barium-137 was 18.0. Detailed descriptions of the counting equipment and standardization procedures used have been given by Lowman et al’ (1958) . 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 col- lection also are given. The levels of cesium-137 found in individual samples ranged between 731 d/m/g dry weight at Kabelle Island, Rongelap Atoll, 28 d/m/g dry weight at Utirik Island, Utirik Atoll. and The individual EP ot pact