The occurrence of Cs-137 in size fractionated samples in river water from the Abukuma River system, (the Kuchibuto and Abukuma Rivers, five sampling events for three sites) was studied from June 2011 approximately some three months after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident until December 2012. The total concentration of Cs-137 (mBq/L) in river water was generally high at the upper stream site in the Yamakiya District within the evacuation/off-limits zone. The Cs-137 concentration was about 1 Bq/L for the first sampling campaign (June 2011) at all sites, but then decreased substantially to about one-tenth of that by the time of a second sampling campaign (November or December 2011). The Cs-137 in the <0.45 mu m fraction was present exclusively as a dissolved species rather than as a species adsorbed on suspended solids or complexed with organic materials. The contribution of the dissolved fraction ranged from 1.2 to 48.9% (averaged 20%) of the total concentration of Cs-137 throughout the observation period. The maximum contribution of Cs-137 was found in the silt size fraction (3-63 mu m), which can be explained by the relatively large K-d values and the suspended solids (SS) concentration of this size fraction. Although the concentration (Bq/g) of Cs-137 ineach size fraction did not show any significant trends and/or variations for any of the sampling campaign, K-d values for each site increased with time. Furthermore, it was found that the K-d values decreased with distance from the headstream in the off-limits zone. Thus, the data acquired in this study give an overview of the radiological situation for Fukushima including temporal and spatial variation of radiocaesium in a natural riverine system, within a few years after the accident. (C) 2014 Elsevier Ltd. All rights reserved.