In the present study, mussel (Mytilus galloprovincialis, Lamarck, 1819) and sediment samples were collected from the Bosphorus strait and the Golden Horn estuary in 2008 and 2009. Activity concentrations of Cs-137, K-40, Th-232 and U-238 were measured using a gamma spectrometer equipped with HPGe detector. Sediment samples were separated into < 63 mu m and > 63 mu m particle fractions. Analysis of radionuclides was carried out on these two fractions of sediment as well as the soft and shell tissues of mussels. Ranges of radioactivity concentrations in the soft parts of mussels were as follows: Cs-137, 0.86-2.43; K-40: 261.1-496.7; Th-232: 0.49-3.58; and U-238: BDL (below detection limit)-1.38 Bq kg(-1) in dry weight. Ranges of radioactivity concentrations in the < 63 and > 63 sediment fractions were as follows: Cs-137, 8.58-67.92 and 1.12-26.40, K-40, 341.4-683.0 and 281.9-662.2; Th-232, 10.97-20.16 and 7.18-19.18 and U-238, 13.97-27.25 and 6.41-18 Bq kg(-1) in dry weight, respectively. The effect of some physical-chemical parameters on the radionuclide accumulation was also examined in the sediment samples. All data in the current study were compared with data in the literature.