A massive amount of radioactive substances, including cesium-137 (137Cs), emitted from the disabled nuclear power plant has deposited on the forested areas in the northeastern region of Honshu Island, Japan after the Fukushima Daiichi nuclear power plant accident. Forests in these regions are particularly important not only for the forest products industry but also for source areas of drinking water and for residential environments. To clarify the mechanisms of diffusion and export of 137Cs deposited on the forested ecosystem, we initiated intensive field observations in a small catchment, including forest and farmlands, in the Kami-Oguni River catchment in the northern part of Fukushima Prefecture. The following expected major pathways of 137Cs diffusion and export were investigated: (1) transportation by water movement of dissolved and particulate or colloidal forms through hydrological processes and (2) diffusion through the food web in terrestrial and aquatic ecosystems of forests. Preliminary findings indicated the following: (1) most of the 137Cs was discharged as suspended matter, and particulate organic matter appeared to be the most important carrier of 137Cs. High water flow generated by storm accelerated the transportation of 137Cs from the forested catchments. Estimation of 137Cs export thus requires precise evaluation of the high flow acceleration during storm events. (2) Because litter and its detritus may form the biggest pool of 137Cs in the forested ecosystem, 137Cs diffusion occurs more rapidly through the detritus food chain than the grazing food chain. Most predators have already ingested 137Cs, particularly in aquatic environments. An urgent question is when and how 137Cs diffuses through grazing food chains and how rapidly this process occurs. To elucidate or predict these phenomena, the mechanisms of 137Cs release from litter and soil organic matter need to be clarified.