The incidence of 6 kinds of gall stone classified according to the mineralogical structure was as follows: Radial structure 60 (26.5%), radial and stratified structure 48 (21.2%), amorphous structure 17 (7.59c), combined structure 5 (2.2%), and special structure one (0.4%). Though the crustal stone was the largest group, the incidence of the radial structure stone was higher than that reported previously. Radial structure and radial and stratified structure were often found in the gall bladder stone. The incidence of the crustal structure stone was much higher in the common bile duct stone compared to the gall bladder stone. The cholesterol contents of the radial structure and radial and stratified structure stone were almost the same. Concerning the shape of the crystal, different from the previous opinions, needle-like crystal in radial structure stones and sheaf-like crystal in radial and stratified structure stones were not infrequently found in the author''s cases. The relation of crystal size with stone structure was statistically not significant. The central cavity was observed in 30 cases (13.3% of all stones) in the microscopic examination and in 14 cases (6.2%) in the X-ray examination. In the crustal structure stone, the conformity lamina was found more often in the gall bladder stone (65.3%) than in the common bile duct stone (47.7%). The incidence of the disconformity lamina was 4.1% in the gall bladder stone and 13.6%, in the common bile duct stone, and that of the unconformity lamina was 14.3% in the gall bladder stone and 20.5% in the common bile duct stone. In sum, both disconformity and unconformity occurred more frequently in the common bile duct stone than in the gall bladder stone. Five cases of combined structure stone were found. In 4 of them the respective components of nucleus and shell were either cholesterol and bilirubin or bilirubin and cholesterol. In one case, a calcium carbonate lamina surrounded a nucleus of bilirubin and its shell consisted of cholesterol. In the special structure stone, a large amount of calcium carbonate was observed. Histological examination of the gall bladder showed that the crustal structure stone was frequently seen in the gall bladder with acute inflammation. Although, incidence of the crustal stone in the gall bladder with chronic inflammation alone was not particularly high, whereas once accompanied by acute inflammation the incidence of the crustal stone increased remarkably. Therefore, it may be concluded that the formation of the crustal structure stone is originated from a colloid chemical change in the bile due to inflammation and in the inflammatory product of the biliary tract and it may be formed in a relatively short period.