The δ 1 8 O values of smectite (+ 14.2 to +20.5‰) from soils and sediments along the coastal plain of Ariake Bay, northern Kyushu, Japan, suggest that the smectites are detrital in origin. Samples of detrital smectite from the catchment of major rivers entering Ariake Bay have similar δ 1 8 O values (+18.2 to +22.8‰). The oxygen-isotope data rule out neoformation at low temperatures during interaction with seawater as a possible origin for most of this smectite.