The formation of gel film of colloidal matter in sendo on the surface of rush stem seems to be highly affected by the clay content and nature of the soil used in the preparation of mud water. So the mechanical composition of typical sendo samples and some physical properties of clay particles in water were studied by the authors. Sendo samples which are produced in Hyogo, Okayama, Hiroshima and Fukuoka prefectures were examined. Mechanical composition, Akashi and Awaji sendos produced in Hyogo are generally fine in texture and the contents of coarse and fine sands are less than 0.2 per cent, while the clay content varies wide from 10.2 to 40.3 per cent and decreases in the following order: Kyu-Akashi sendo＞Shin-Akashi sendo＞Awaji sendo. Kyushu sendos are also fine in texture but Hiroshima sendos are generally coarse in texture. And Okayama sendos can be devided into two groups, the fine and the coarse ones. Dispersion characteristics. Most of Shin-Akashi and Awaji sendos highly disperse in water, while Kyu-Akashi sendos coagulate completely in water. It is found that the latter contains a considerable amount of electrolyte, mostly of calcium sulfate. Hence, the difference in dispersion might be due largely to the content of electrolyte in sendo samples. Most of Hiroshima and Okayama sendos are also highly dispersive in water, while Kyushu sendos show a markedely different dispersiveness. There are sharp distinctions in the dispersion ratio, that is, the ratios of Kyu-Akashi and clayey Okayama sendos are very low as compared to the other ones. The indices of swelling increase according to the following order : Kyu-Akashi sendo＞Kyushu sendo＞Shin-Akashi sendo＞Awaji sendo＞Hiroshima sendo. Collapse characteristics. The collapse characteristics of clod in water determine the ease of preparation of mud water. Kyu-Akashi clods collapse easily and rapidly in water forming leaf-like flakes, while Shin-Akashi clods do not collapse at all, but break into only several pieces in water. Hence, a marked difference is found in the degree of collapse between the two groups of Akashi sendo, namely the former from 27.0 to 60.5 per cent and the latter from 5.3 to 16.5 per cent. The two types are also found among Awaji sendo samples, while Hiroshima, Kyushu and Okayama sendos collapse easily in water forming fine particles or granules, but the degrees of collapse are found in a rather wide range from 15.1 to 95.6 per cent.