There have been few reports on mosquito fauna in Karuizawa Town. The objective of this study was to clarify the species composition of mosquito fauna, using 3 CDC traps and 3 Ovitraps, and collecting mosquito larvae near these traps in Karuizawa Town, the Seishoji Temple and the Nagakura residential area. I collected 95 individual mosquitos representing 3 genera and 6 species during the investigation periods. Culex pipiens complex was the dominant group in Karuizawa Town.