In this study, the activity concentrations of naturally occurring U-238, Th-232 and K-40 and anthropogenic Cs-137 radionuclides were measured for 56 soil and beach sand samples collected from the Princes' Islands (Istanbul) in the Sea of Marmara, using a gamma-ray spectrometer. In each case, the outdoor gamma-ray dose rates in air were measured at the coordinates where the samples were collected, using a portable gamma-ray detector. In addition, the Po-210 specific activity concentrations were measured for 13 different types of fish and 3 black mussel samples that are consumed in the region, using an alpha spectrometer. The mean activity concentrations of natural U-238, Th-232 and K-40 and artificial Cs-137 radionuclides were calculated to be 33.8 +/- 0.5 Bq kg(-1), 19.3 +/- 0.3 Bq kg(-1), 429.5 +/- 4.5 Bq kg(-1) and 8.9 +/- 0.1 Bq kg(-1), respectively, in the soil and beach sand samples. Activity concentration contour maps of the Princes' Islands (Istanbul) were constructed. The mean annual effective dose values for natural and artificial radioactive nuclide activity concentrations were determined to be 55.4 mu Sv y(-1) and 1.2 mu Sv y(-1), respectively. The average outdoor gamma-ray dose rate measured in the air was 0.68 mSv y(-1). The mean activity concentrations of Po-210 in the fish and mussel samples were measured to be 16.2 +/- 1.9 Bq kg(-1) and 516.1 +/- 25.8 Bq kg(-1). The average annual effective ingestion doses due to Po-210 from the consumption of these seafoods were calculated to be 106.9 mu Sv y(-1) and 619.4 mu Sv y(-1), respectively. The results of this study were compared with the limits proposed by UNSCEAR, ICRP and the values obtained in other similar studies worldwide.