The fluorine content of 67 samples of mineral waters in Misasa, Asozu, Togo, Hamamura, and Iwai Hot Springs was investigated by Zirconium-Alizarine Sulfonate method. It ranged from 1.5 to 15 mg. per liter, 3-6mg. per liter in the majority. Most of the river and well waters in the same districts contained no measurable amount of fluorifle． In Misasa, Asozu, Togo and Hamamura Hot Springs 68-83 per　cent of the schoolchildren who take thermal waters habitually as drinking water suffer from mottled tooth. On　the contrary 2-10 per cent of the schoolchildren who do not drink thermal waters habitually showed mottled tooth. In Iwai no inhabitant uses thermal water as drinking water, so that the incidence of mottled tooth among them was only 2.8 per cent. Concerning the incidence of struma in the schoolchildren no relationship was proved between the occurrence of mottled tooth or fluorine content of thermal waters.