The bioaccumulation of polonium in the Baltic mussel Mytilus trossulus was studied experi-mentally in an attempt to discover the differences between food and water as sources of polonium. The 209Po isotope was used as tracer for accumulation and excretion. The highest 209Po con-centration was found in the hepatopancreas of mussels exposed to spiked water for 24 days. Both parts of the experiment (exposure to food and water) showed that while the rate of bioaccumula-tion of polonium by Mytilus trossulus was very high, the excretion rate was also relatively high. Furthermore, in both cases the duration of the accumulation-excretion (a-e) cycle of polonium in Mytilus trossulus was 10 days. Finally, a simple mathematical model approximating the experimental data was developed.