Soil erosion is a major degradation process in watersheds. Inappropriate land use is the main reason for soil loss by erosion and for the associated sedimentation of lakes and reservoirs in Turkey. Soil erosion rates are usually part of the information to be considered for land evaluation under special conditions and they are generally estimated using empirical or theoretical models. Fallout radionuclides (FRN's) provide a powerful new technique for obtaining empirical information on erosion rates that can be used for predicting the impacts of proposed land management strategies on soil erosion and sediment yield within river basins. The Universal Soil Loss Equation (USLE) is one of the most comprehensive methods for estimating soil erosion, but the USLE does not estimate deposition. The aim of the study reported in this contribution was to compare soil losses estimated using Cs-137 measurements and the USLE at the Pasakoy and Esenceli study sites in the Omerli dam catchment, which supplies drinking water for Istanbul city. Soil losses estimated usin Cs-137 were 9.62 and 3.32 t ha(-1) yr(-1) for upper slopes and 10.34 and 2.19 t ha(-1) yr(-1) for mid-slopes, in Pasakoy and Esenceli, respectively, while those calculated using the USLE were 11.53 and 16.21 t ha(-1) yr(-1) for upper slopes and 19.58 and 14.45 t ha(-1) yr(-1) for mid-slopes, in Pasakoy and Esenceli, respectively.