The layer-silicate clay mineralogy of paddy soils of the regions of Chikugo-sankan and Chikugo-heiya situated in the southern part of Fukuoka prefecture, Kyushu, was investigated. The clay mineralogical composition of the paddy soils was well correlated with the geological characteristics of the sediment from which the paddy soils were derived. The paddy soils distributed along the Chikugo river, the biggest river in Kyushu, were characterized by the presence of smectite. The paddy soils along the Yabe river and at the foots of the Minou and Yame mountains composed of crystalline schist contained relatively high amounts of chlorite and clay mica. The paddy soils at the foots of mountains composed of andesite or granite and on the Pleistocene terrace were dominated by kaolin mineral. The mineralogical difference of the soils can be related to the difference in the natural soil fertility and soil buffering ability, and was reflected in the highest rice grain yield of the paddy soils along the Chikugo river.