y Germanium (HPGe) detector was used to measure natural (Ra-226, Th-232 K-40) radioactivity levels in sediment and seawater samples, and soft tissues of Mediterranean mussels (Mytilus galloprovincialis) of three different sizes, collected from twelve different stations along the coast of Giresun, Trabzon, Rize and Artvin provinces in Eastern Black Sea Region. The average activity concentrations of the sediment and seawater samples collected were 12.65 Bq kg(-1) and 3.57 mBq L-1 for Ra-226, 14.25 Bq kg(-1) and 4.01 mBq L-1 for Th-232 and 294.8 Bq kg(-1) and 317.8 mBq L-1 for K-40, respectively. The activity concentrations in soft tissues of three different sized mussel samples altered from 2.45 to 9.59 Bq kg(-1) for Ra-226 2.22-8.59 Bq kg(-1) for Th-232 and 251.9-382.1 Bq kg(-1) for K-40. Moreover, sediment distribution coefficients(K-d) and bioaccumulation factors (BAF(w) and BAF(s)) in the mussel samples were calculated. While the bioaccumulation of Ra-226 and Th-232 from seawater to mussel was observed, the bioaccumulation K-40 was observed from sediment to mussel. The ratio of the average concentration of radionuclides in large-sized mussels to that in water and sediment is greater as compared with smaller mussels. Although there were statistically differences (p < 0.05) among mussel sizes according to Th-232 levels for activity concentrations, BAF, and BAF, values in soft tissues of the mussel samples, it wasn't observed difference according to Ra-226 and K-40 levels (p > 0.05).