ABSTRACT Radiocesium and stable potassium levels were determined in samples of muscle tissue of Birgus latro, the coconut crab, collected at Rongelap Atoll, Marshall Islands, during March and August,1958, and March 1959, and at Utirik Atoll in March 1959. Levels of cesium-137 ranged between 731 Kabelle Island, Rongelap Atoll, d/m/g dry weight Utirik Island, Utirik Atoll. at The average potassium value for all samples was 13.05 mg/g dry ation of 3.66. and 28 d/m/g dry weight at weight with a standard devi- No significant correlation between cesium-137 and potassium levels was found. There was no significant difference in the average levels of cesium-137 in crabs collected at different times at the same island.