from the leaves of the vegetation and perhaps also to some — shielding effect by additional moisture in the soil. It was certainly not due to rapid vertical movement of material in the soil. Analysis of soil leachates and soil cores shows that vertical movement of radionuclides in atoll soils is very slow. For example, cores taken in immature soils in 1963 still contained 90 per cent of the activity in the top centimeter. However, the reduction of gamma-dose rates to approximately half the predicted levels in 1959-63 probably reflects the downward movement of the long-lived gamma-emitter cs??? in the Soil. Cesium-137 and spt?5 are very slowly leached in the atoll s0ils while other gamma-emitters, cel44 _p144 | Eut?5, gn®?, coo? and Mn°“ tend to remain at the surface (1+6+7) | Although the gamma-dose rate values at Rongelap islet in 1959-63 fall on the theoretical curve, it appears that the levels due to the 1954 fallout have fallen below values predicted by the theo- retical curve here also. Since the theoretical curve had reached background levels by 1959 it would be expected that the sum of gamma-dose rates due to the fallout and due to background would be approximately twice background. CONCLUSICNS The Rongelap experience has shown that the decline of a) gamma dose rates can be approximated from the decay curve for