The vertical distribution of radionuclides in the bottom sediment of Gaevle Bay, southern Bothnian Sea, was investigated after the Chernobyl fallout. Sediment cores collected two years after the initial fallout revealed the highest radionuclide concentrations found in the Baltic Sea, reflecting the heavy fallout over this region. Concentrations of nuclides were almost constant down to about 8 cm sediment depth. Results from calculations by solving the one-dimensional diffusion equation were compared with the measured concentrations of radionuclide. The computed values were much lower than the observed ones, and diffusion alone could not account for the distribution pattern in the sediment. The macrobenthic community was scarce and vertical mixing of pore water could not be explained by bioturbation either. Vertical displacement of the pore water, caused by changes in density of the overlying water column, is a possible mechanism which could explain the rapid penetration of fallout nuclides in the soft bottom sediment. 8 refs, 16 figs, 3 tabs.