An additional factor which may accentuate the condition found in Birgus is currently under study at this Laboratory in cooperation with the Department of Botany, University of Washington The low potas- sium levels in the soils at Rongelap represent a potassium for at least some of the plants. Greenhouse experiments deficiency in which Rongelap soil was used as well as field trials at Rongelap have demonstrated that potassium fertilization decreases cesium-137 uptake plants (Walker and Held, 1959). by This observation is in agreement with the reported increase in cesium-137 uptake by plants in other soils as the potassium concentration in the soil is reduced by prolonged cropping (Nishita et al., 1953 and i959). It appears possible that if a surplus of potassium were present in the soils at Rongelap, potassium levels in Birgus muscle would be more uniform and cesium-137 levels lower. The simplest system studied at Rongelap with respect to cesium-137 and potassium was the soil. Cesium-137 and potassium in rain water which had percolated through undisturbed soils were found to be present in approximately proportional amounts under changing conditions of precipitation and fertilization (Cole et al., or higher trophic level, 1959). However, at the second represented by Birgus, if any such :elationship exists between the uptake of cesium and potassium, it is masked by several unknown variables in the ecosystem. ao” yo”