Studies on seasonal and annual fluctuations in mosquito population size were carried out at 5 Sis (Cities) and 13 Guns (Counties) of Cholla-bugdo (Province) in 1985~1990. Mosquitoes were collected weekly throughout the main mosquito season (the 4th week of Mayo the 1st week of October) at each study site, at which a light trap was set up in a cow shed and operated overnight. The results are summarized as follows: (1) Even though Culex tritaeniorhynchus population densities showed severe fluctuation by locality and/or by year, the overall size in Cholla-bugdo was not much changed during the study years, except 1989 when it was much higher. In case of Anopheles sinensis, seasonal and annual fluctuations in population size were somewhat greater than Cx. tritaeniorhynchus, showing no tendency towards any progressive increase or decrease. (2) Among 18 study areas, high population size of Cx. tritaeniorhynchus was shown, in the order of their size, at Oggu-gun, Jeongub-gun, Namwon-gun and Gochang-gun. Low population at Muju-gun, Jinan-gun, Jangsu-gun and Imsil-gun was shown, all of which are mountainous areas with smaller acreage of rice fields. In the case of An. sinensis, high population was observed at Namwon-gun, Gochang-gun, Oggu-gun, Jeongju-si and Jeongub-gun, and low population at Muju-gun, Igsan-gun, Gimje-gun and Wanju-gun, most of which are plain area. (3) Cx. tritaeniorhynchus population began to appear during the period of 1st~3rd week of June, increased with a gentle slope in population size until 2nd week of August, and immediately followed a steady rise. Showing the peak at 5th week of August, population size kept declining rapidly In the case of An. sinensis, population size was very steadily increased from 3rd week of June, reached the peak after two weeks (1st week of July), followed by a significant decline till 1st week of August, and showed a small secondary peak at 4th week of August. (4) Besides Cx. tritaeniorhynchus and An. sinensis, seasonal and annual fluctuations in population size of Cx. pipiens pallens, Cx. vagans, Aedes vexans nipponii, Ae. albopictus and Amigeres subalbatus were also observed and discussed.