Adults mosquitoes of Culex tritaeniorhynchus were collected by light traps in a pigsty at Kochi City located in the central Kochi Prefecture and that at Tosashimizu City located in the southern Kochi Prefecture from 1965 to 1972. A marked difference was observed in seasonal fluctuation of mosquito population between the two sites of collection. During the period of observation, the incidence of Japanese encephalitis patients fluctuated in parallel with the average population of mosquitoes collected from June to September. The first incidnece of Japanese encephalitis patients in Kochi Prefecture generally starts from south-western Kochi Prefecture including Tosashimizu City and gradually moves north. This is coincident with the trend observed in the appearance of the first mosquito population in these area.