Physical-chemical form of radionuclides is an essential factor to decide the fate of radionuclides in the environment, and also play an important role in their migration in soils. Cesium-137 is an important radionuclide for the assessment of radiation exposure to the public around the nuclear facilities because of its high fission yield and relatively long half-life. In order to investigate the physical-chemical forms of fallout ¹³⁷Cs (mainly derived from nuclear weapons tests from 1940s to 1980s) in cultivated soils, ¹³⁷Cs and stable Cs (¹³³Cs) extracted with two extractants were determined. Residue after the stronger extraction was separated on the particle size, and then analyzed for ¹³⁷Cs and stable Cs. Soil samples were collected in 1991-1994 from agricultural fields throughout Aomori Prefecture, Japan, where the first commercial nuclear fuel reprocessing plant and a fission reactor are located. Two fractions were individually extracted with 1M CH₃COONH₄ solution (exchangeable fraction, E2), and 0.8M CH₃COONH₄ in 5% HNO₃ solution after H₂O₂ oxidization (E2+organic bound fraction, E4). The residues after extracting the E4 fraction were divided into four size fractions; clay (<0.002 mm), silt (0.002-0.02 mm), fine sand (0.02-0.2 mm) and coarse sand (0.2-2 mm). The silt fraction was not recovered because of incomplete separation of silt from clay. The concentration of ¹³⁷Cs in the fractions and total content (E5) was determined using a low background 2p gas flow proportional counter after radiochemical separation of Cs or a well-type Ge gamma-ray detector. The concentration of stable Cs in the samples was determined by ICP-MS after adequate pretreatment. The distribution pattern of ¹³⁷Cs contents among the fractions were different for the soil samples, and has no correlation with soil properties including pH, a cation exchange capacity (CEC) and a carbon content. The content of ¹³⁷Cs in the exchangeable fraction was less than 20% (4-18%), and that in the organic bound (E4-E2) and strongly bound factions (E5-E4) were 0-57% and 25-96%, respectively. The concentration of ¹³⁷Cs in the clay fraction was several folds higher than that in other size fractions. The specific activity of ¹³⁷Cs (the ratio of the concentration of ¹³⁷Cs to that of stable Cs) decreased in the order of the exchangeable, organic bound and strongly bound fractions. Equilibrium between fallout ¹³⁷Cs and stable Cs has not reached among those fractions, even though most of the ¹³⁷Cs had been deposited on the soils more than several decades ago. This work was supported by a grant from the Government of Aomori Prefecture, Japan.