Uranium series nuclides in the Tertiary arkosic sands from a high rainfall area in Japan were determined and their characteristics and behaviour were studied. Uranium was concentrated in the upper (c. 241 ppm) and lower (c. 581 ppm) layers although no specific U mineral was observed. The filtration experiment showed that U in the groundwater is in the dissolved form rather than colloidal or particulate forms. Most U in the concentrated layers was in easily soluble fractions (such as AcONa/AcOH soluble) while the U content in the water around the site was low. The fact that the 230Th/234U activity ratios of a sample in the upper layer were much greater than unity suggests that U migration occurred within 0.3 Ma. However, the 234U/238U and 230Th/234U activity ratios of samples in the lower layer were close to unity, suggesting that the material has not been strongly leached. Using chemical leaching techniques, a small disequilibrium among the fractions is observed, even though the bulk sample was near equilibrium. It is inferred that sorption of U is more effective than precipitation in this area.