The authors have collected mosquitoes with light-trap twice a week from April to December, 1959 at Okayama city. Species, number and seasonal distribution of the mosquitoes collected in 1956 were practically the same as those of the average year, while the encephalitis cases reported from within the city area were 337 as against 309 in 1950, the peak of the endemicity following as usual that of activity of the mosquitoes involved.