Summary The five known endemic areas of schistosomiasis in Japan were surveyed by the Commission on Schistosomiasis with the view of ascertaining the extent of these areas and gathering information which would be pertinent to the prevention of this disease in military personnel. The methods employed in the surveys included the compiling of all available information from national, prefectural, and local health authorities, the collection and examination for schistosome eggs of stool samples from children in certain representative schools in each area, and the collection and examination of Oncomelania nosophora, the snail intermediate host of Schistosoma japonicum, in an effort to determine its distribution within the area and the rate of infection with cercariae of the parasite. The endemic areas surveyed were as follows: (1) The Tone River area in Chiba and Ibaraki Prefectures. (2) The Kofu area in Yamanashi Prefecture. (3) The Numazu area in Shizuoka Prefecture. (4) The Fukuyama area in Hiroshima and Okayama Prefectures. (5) The Kurume area in Saga and Fukuoka Prefectures, island of Kyushu. Individuals found infected with S. japonicum on stool examination have been reported from other areas including Tochigi, Aomori, and Fukui Prefectures. National health authorities were of the opinion that such cases do not represent infection acquired in these Prefectures but rather imported infections. A relatively large number of such cases has been reported from Fukui Prefecture (1) but it was the view of Japanese health officials and parasitologists that these reports are either in error or represent cases of infection acquired elsewhere. In reply to a specific inquiry, the Health Officer of Fukui Prefecture stated that there were no cases of the disease in that Prefecture. Schistosomiasis was formerly endemic in certain parts of Tokyo Prefecture, but it is said that all these foci of infection have now been eradicated. The Tone River area, which lies partly in Chiba Prefecture and partly in Ibaraki Prefecture, apparently has the lowest infection rate of any area in Japan. While the disease is still endemic in certain parts of this river valley, it is at a very low ebb. An examination by the commission of 390 school children in six localities revealed only 3 cases of the disease. The commission was unable to find specimens of O. nosophora in any place in which a search was made, although the snail undoubtedly still occurs in certain sections. However, the entire river valley east of the junction of the Kinu River should be regarded as a possible endemic area. From the standpoint of incidence of infection and morbidity rate, the Kofu area, Yamanashi Prefecture, is the most important one in Japan. Of 458 children from four schools in which examinations were conducted by the commission, 245, or 53.5 per cent, were found infected. In spite of the fact that a control campaign has been aggressively carried on in this area since 1942, it is apparent that little has actually been accomplished in the reduction of the incidence of infection or the number of clinical cases. The Numazu area in Shizuoka Prefecture covers only about 10 square miles and is the smallest focus of the disease in Japan. Only 9 per cent of 155 children from a school in the center of the area were found infected. Authorities stated that there has been a marked decrease in the number of clinical cases of schistosomiasis in this area since the drainage of a large swamp in the heart of the area in 1942. The Fukuyama area in Hiroshima and Okayama Prefectures ranks third in importance of the endemic areas in Japan. Examination of 357 school children in the area indicated an infection rate of 9.5 per cent. Through the organized control work which has been carried on in this area over the past 30 years, the disease has been gradually reduced and the distribution of the snail intermediate host considerably restricted. At the time of the present surveys, the infection seemed to be confined principally to the townships of Kannabe and Miyuki in Fukayasu County of Hiroshima Prefecture, with the center of the focus near the village of Katayama where the disease was first discovered. The Kurume area in Saga and Fukuoka Prefectures, island of Kyushu, is approximately of the same size as is the endemic focus at Kofu, Yamanashi Prefecture. However, the infection is more spotty than in the Kofu area, some sections showing a high incidence of the infection while in other sections the disease is at a much lower ebb. Examination of 328 children in three schools within the area revealed an infection rate of 25.9 per cent. The distribution of the snail intermediate host within the area varies considerably, the snails being found in abundance in the high incidence zones and few in number or absent in certain other sections where the incidence was found to be lower. This situation is apparently due to the operation of natural factors since practically no control work has been carried out in this area. A total of 1,688 school children between the ages of 8 and 15 years was examined by the commission in the five endemic areas. Of these, 381, or 22.6, were infected with S. japonicum. Due to a confusion in records, the sex of 14 individuals was not recorded; one of these individuals was positive. Of the remainder, 802 were males, of whom 234, or 29.2 per cent were infected, while 872 were females, of whom 146, or 16.7 per cent, were found positive. This considerable difference between the infection rates in males and females confirms the impression of Japanese health authorities and physicians that males more frequently suffer from the disease due probably to their greater opportunities for acquiring infection.