A series of incubation experiments was performed to investigate the release of caesium from either undisturbed or resuspended freshwater sediment which had not been previously dried. Ammonium and potassium ions readily released caesium from resuspended material, in keeping with the results of other workers, but treatment of the overlying water with hydrochloric acid (pH 4) or nitrilo triacetic acid did not enhance release of caesium from undisturbed sediment. Caesium was released from undisturbed sediment when it was treated with 0·5 mol litre−1 NaCl, but the release of 0·5% (as dry weight of sediment) was low compared with the release which occurs from resuspended sediment treated with lower concentrations of sodium chloride. The smaller release observed in the experiments with undisturbed rather than resuspended sediments was attributed to the interstitial waters being buffered by redox and solid phase reactions. There was minimal release (0·5%) from undisturbed sediment after 40 days of either oxic or anoxic incubation, but the magnitude of the release was consistent with assumed ammonia concentrations in the pore-waters of the surface sediments and experiments which investigated the ammonia-induced release from resuspended wet sediment. It was suggested that the observed elevated contributions in the bottom waters of lakes could be largely due to remobilization from sinking particles rather than release from the sediment. The results indicate that the caesium present in lacustrine sediments is unlikely to be appreciably remobilized in response to acidification, eutrophication or occasional industrial accidents. However, inundation of coastal freshwater basins by seawater could reintroduce into the aqueous environment considerable burdens of formerly safely bound caesium.