The seasonal distribution of mosquitoes in Okayama Prefecture was investigated in 1955 by means of light-trap at Niizato, Niimi, Takahashi, Soja, Kurashiki, Tamashima and Okayama in connection with the incidence of Japanese B encephalitis. Culex pipiens, Culex tritaeniorhynchus and Anopheles sinensis were collected through the prefecture, more abundantly in the southern district than in the northern mountainous district. Culex tritaeniorhynchus appeared in enormous numbers during the hottest season from late July through August after comparatively cool days of early summer. A majority of infections (77 cases in all) with Japanese B encephalitis has been reported during the period of August, though early September, with the infection rate corresponding with the distribution of the mosquitoes. It is to be noted that the incidence of the encephalitis and the distribution of the mosquitoes, especially of C. tritaeniorhynchus in Okayama Prefecture are parallel to each other, and may be graded into three (north, middle and south) zones according to their respective intensity, in exact conformity with the isothermal geographical zones.