ABSTRACT The redistribution of radionuclides in atoll soils following fallout from a nuclear device is described. The soils are cal- careous, containing no inorganic colloids, and their exchange capacity is directly related to organic content. Comparison of gamma-ray spectra of depth increments from young and old soils shows that Cs 137 and Sb 125 : . ' : move most readily in old soil, while the principal gamma-emitting radionuclide moving in young soil : 125 is Sb - sr 90 moves in both old and new soils, and quantitative differences in vertical movement between soil types is obscured by the highly variable surface distribution of the radionuclides. There is a vertical gradient in the distribution of radionuclides even within the surface inch. Litter redeposits est37 and sr?° at the soil surface and bird droppings have added zn°? and co°?, In young soils the highest levels of radioactivity are associated with soil algae found as a surface crust in undisturbed areas and in coral fragments in eroded areas. Horizontal movement is localized and probably is of little overall importance. Buried organic horizons contain more cst?” than adjacent soil layers, and roots are generally more radioactive than the surrounding soil except at the soil surface. Pumice particles in the soil adsorb radionuclides but pumice is found infrequently. Mechani- cal mixing by animals in old soils and by erosion in young soils