The Golden Horn is an estuary located in the center of Istanbul receiving freshwater discharges from two creeks and connecting to the Bosphorus Strait. Activity concentrations of natural and artificial radionuclides were determined in mussels (Mytilus galloprovincialis) and sediments from the Golden Horn sampled in February 2012. Mean activity concentrations of Cs-137, K-40, Ra-226, Ra-228, Po-210 and Pb-210 in the mussels were determined at 1.03 +/- 0.23, 389 +/- 41.6, 2.61 +/- 1.23, not detected (ND), 91.96 +/- 37.88 and 11.48 +/- 4.85 Bq kg, respectively. In sediments, it was observed that Cs-137, K-40, Ra-226, Ra-228, Po-210 and Pb-210 activity concentrations in <63 mu m particle fraction of sediment were generally higher than those determined in mussels. Po-210 and Po-210/Pb-210 ratios in mussels from the Golden Horn were much lower than in mussels from other coastal regions and this was related to low plankton productivity and eutrophication of the Golden Horn. (C) 2014 Elsevier Ltd. All rights reserved.