The following researchs have been carried out on the behavior of Culicoides arakawae, the intermediate host of leucocytozoon infection of the chicken. From 1961 to 1964, C. arakawae have been collected by a light trap once or twice every week at the hen house of our laboratory and a farmer's hen house at Hataayu, Makiishi in Okayama, city. The following items were also investigated: 1.)the successive change of the number of C. arakawae flied into the hen house in the evening, 2.)the number of C. arakawae caught by an insect net at various heights, 3.)the successive change of the number of C. arakawae caught by a light trap placed in the hen house. The results obtained were as follows. 1. C. arakawae comes flying into the hen house when the mean temperature is higher than 10℃. But, when the mean temperature is higher than 10℃, there are no clear correlation between the temperature, humidity and weather and the number of C. arakawae caught by light trap. The sex ratio of C. arakawae caught by a light trap varies with temperature, and the higher the temperature is, the greater is the percentage of the female. The ratio of engorged females to the whole females also varies with temperature. When the temperature is lower than 20℃, the percentage of engorged females is almost always below 50. When the temperature is higher than that, the percentage is from 70 to 90. 2. The number of C. arakawae caught by a light trap placed in the hen house shows the normal distribution with the central axis at 2 am. 3. It is probably certain that C. arakawae does not hibernate in the imaginal stage in the neighbourhood of Okayama city. 4. The number of C. arakawae come flying into the hen house in the evening, is most numerous during one hour following sunset. And in the summer season, in the flies caught by a light trap placed in the hen house, the percentage of females is usually far greater than that of males, but the sex ratio of C. arakawae, come flying into the hen house, is about 1:1. 5. C. arakawae usually flies below 20 m on the ground.