Clay fractions (< 2 p) from f our muds in the Ariake Sea and one soil derived from the marine deposits in the Kojima Bay were examined by x-ray, chemical, thermal, and electron micrograph methods. Besides these analyses ethylene glycol retention and base exchange data were presented. The results obtained indicated that the clay minerals of these samples were predominantly constituted from minerals of the illite-montmorillonoid series. This montmorillonoid may be somewhat iron rich montmorillonite or some intermediate members between the montmorillonite and the nontronite. And the clay separates from the Kojima Bay showed more illitic behaviour than the one from the Ariake Sea. The striking similarity of these clay fractions examined, without regard to the locality, suggests that these clays were formed in a similar environment of the shallow sea.