The biophysical behaviour of the radioactive corrosion products 51Cr, 54Mn, 58Co, 60Co, 65Zn and 110Ag m has been studied in the marine environment up to 150 km from a nuclear power station using the brown seaweeds Fucus vesiculosus and F. serratus as bioindicators. The activity concentration in Fucus per unit activity released from the nuclear power station is about equal for 54Mn, 58Co, 60Co and 65Zn but lower by a factor of 3 for 110Ag m and by a factor of 10 for 51Cr. The biological half-time of radioactive cobalt in naturally growing F. vesiculosus was 60 ± 15 days during the summer. Approximately the same value was found for 54Mn and 65Zn. Our results show that 85–90% of the activity released from the nuclear power plant follows the dominating north-bound water stream and 10–15% is transported southwards. For 54Mn, 58Co and 65Zn the decrease in activity concentration C(z) with distance, z (km) north of the power plant can be described by a power function, C(z) = α.z −1·4, where α is a constant.