Polonium-210 has been measured in Danish fish meat caught in the North Sea, the Kattegat and the Baltic in 1991-1994. Average values of 0.35, 0.65 and 0.96 Bq Po-210 kg(-1) fresh weight were observed for cod, herring and plaice fillets, respectively. The difference between species is statistically significant, whereas no effect of salinity could be observed. There is a high variation giving SD values in the range 70-100%. Mytilus edulis soft parts were analysed for Po-210 from II Danish locations ranging from full North Sea salinity to Baltic 8 parts per thousand water. Significantly increasing Po-210 concentrations with a decreasing 'condition index' were observed. Concentrations at two former phosphate industry sites were not statistically different from the other locations. The average Po-210 concentration in the M. edulis soft parts was 149 Bq kg(-1) dry +/- 55% SD (n = 41). The present levels of Po-210 in fish and mussels may represent a natural baseline. Copyright (C) 1996 Elsevier Science Ltd.