The overwintering ability of eggs of 3 temperate and 2 tropical strains of Aedes albopictus was assessed at 5 field sites during winter 1991-92 and 4 field sites during winter 1992-93 in Japan. Nagasaki, the warmest site, is the only site at which tropical Ae. albopictus could possibly become established. The eggs of temperate Ae. albopictus can survive less than half of the winters in Sapporo. Temperate Ae. albopictus can survive at least some of the winters at Bijo Daira, Kitakami, Nagasaki, Tokyo, and Toyama.