[en] The cesium (¹³⁷Cs) concentrations were measured in the soils and sediments of 14 watersheds, 7 cultivated and 7 noncultivated, in the North Central United States. The ¹³⁷Cs concentration in watershed soils ranged from 56 to 149 nCi/m², with cultivated watersheds averaging 75 nCi/m² and noncultivated watersheds averaging 104 nCi/m². The ¹³⁷Cs concentration in the reservoir sediments ranged from 74 to 1,280 nCi/m², with a mean of 676 nCi/m² for the cultivated watersheds and 365 nCi/m² for the noncultivated watersheds. The ¹³⁷Cs concentrations per unit area in sediments were 0.8 to 18.7 times greater than those found in the contributing watershed soils. This indicated that some ¹³⁷Cs is moving within the watersheds and that the reservoirs are acting as ''traps'' or ''sinks.'' The factors accounting for the variation in ¹³⁷Cs concentration in the soils and sediments of the watersheds are (i) the erosion potential of the watershed, (ii) the sites for adsorption of ¹³⁷Cs, and (iii) the input of radioactivity into the watershed