The concentration of 137Cs from fallout was determined for the soft tissues of Rangia cuneata from the Trent‐Neuse estuary during 1965–1967. The 137Cs concentration in Rangia varied inversely as a logarithmic function of salinity in the estuary, but with a lesser salinity dependence than reported previously for 137Cs concentration factors in Rangia. An increasing concentration gradient of 137Cs with increasing salinity in the estuary was therefore hypothesized and was confirmed by analysis of 137Cs in estuarine water of varying salinity. Transport of 137Cs on sediments into estuaries, coupled with ion‐exchange displacement of 137Cs from the sediments at elevated salinities, is proposed to explain the concentration gradient of 137Cs in the estuary.