An endemic infectious disease, called Tsutsugamushi disease (Kedani fever, river fever), occurs in Japan along the rivers in the northern provinces. In spite of the persistent efforts of numerous workers, its etiology has been very difficult to determine. This peculiar disease has a close resemblance to Rocky Mountain spotted fever in America, but it is transmitted by a different insect. The carrier of the Japanese disease is a minute red mite, abundant in the infested region, that is, in the low lying cultivated fields along the Shinano river, between the riverside and embankments which vary up to one and a half miles from the river. These fields are at times during the summer covered by overflow water. In nature the mite is ectoparasitic in the ear of field mice, but it freely attacks human beings as well as other animals whenever accessible. Zoologically, it is considered Leptus akamushi (Brumpt), a species distinct from the well known L. autumnalis of Europe. The early clinical course of the disease is marked by a gradual rise of the body temperature, a characteristic lesion at the site of the bite, and swelling of adjacent lymph nodes. The incubation period is about eight days. The fever, which is usually 38?-39? C. in the early stage, may rise to 40? C. or even higher, and usually lasts three or four weeks. Complete recovery requires a month or more. In bad cases a fatal result follows about ten days after the onset. The mortality of the disease is approximately forty per cent.