Although the leptospirosis in Ehime Prefecture is well-known and Minami-Uwa-Gun is particularly well-known as the prevailing district of this disease, no large scale examination is as yet reported. The author studied at first the endemic and the type of leptospira all over Ehime Prefecture, and then studied those in Minami-Uwa-Gun, the most heavily contaminated district. In parallel with this, the occurence of the vitreous body opacity in the leptospirosis was also studied. The following is a summary of the results obtained in this study: Ⅰ. Epidemiological study in Ehime Prefecture 1) Gosho district contains about 50% of all the cases of Weil's disease in Ehime Prefecture, and the distribution decreases in the sequence of Mibugawa, Yawatahama, Matsuyama and Uwajima districts. On cold plateaus and in intermountain regions, incidences are few. In Gosho district, the most heavily contaminated district, the soil pH is slightly acid. 2) In Ehime Prefecture, the average temperature of a year is 15°C, the humidity is over 20% from June to September and the amount of rainfall from April to September is over 200 mm; a very good condition for the prevalence of leptospirosis. Incidences are the most in September. 3) This disease is more among the people of strong constitution with high working power, and is also more in the male than the female. 4) The cases of this disease are the most in farmers and the next in fishermen; the cases of each of these two are nearly 50% of all cases. 5) Some interrelation exist between the leptospirosis of a family and that of its domestic animals. Ⅱ. The leptospirosis in Gosho Public Health Office jurisdiction 1) About 50% of the cases exist in Nishi-Umimachi district, and less than this but pretty many cases are observed in Minami-Uchiumi, Shirobe, Higashi-Sotoumi and Gosho districts. 2) The prevalent regions are terraced fields and Rias type sea shores, which are the characteristics of the prevalent regions all over Japan, and especially fishing villages are wet and nasty and are massed by houses, giving a very good condition for the prevalence of the leptospirosis. 3) The climate of this district is that of the subtropical zone; wet and the average temperature of a year of 18℃. 4) The morbidity rate was 1.0% of 7350 people tested. 5) As for the type of leptospira, Weil and Akiyami types are rather many, Particularly Akiyami C type is many, which is usually considered rather less. 6) The Akiyami C strain isolated from the dog is the third one of this strain isolated in Japan. Ⅲ. The Antibody distribution in Ehime Prefecture 1) Of 511 dogs tested, 105 (20.5%) showed a positive reaction, and the positive reaction rate was higher in southern districts. The type of the agent was L. canicola, Weil and Akiyami C. 2) Of 663 horses and tattles, 137 (23.5%) showed a positive reaction, and the positive reaction cases are many in Gosho and Imaharu districts. As for the type of the agent, Akiyami B was the most and the next was Weil, Akiyami A and Akiyami C. The interrelation between the vitreous body opacity and the leptospirosis Of 32 patients with vitreous body opacity, 17 cases (56.3%) showed a positive serum agglutination reaction. Of all the types of the agent found in these patients, each of weil and Akiyami types occupies about 50%, and, of the latter, Akiyami C type was about 25%. The vitreous body opacity appeared about 8 months after the onset of leptospirosis.