Aedes albopictus (Skuse), a mosquito vector of the dengue fever virus, is prevalent in Japan, distributed widely in Honshu Island with its northern limits between latitude 38 degrees to 40 degrees north. The factors affecting distribution of the species in the northern part of Japan were studied using the geographical information system (GIS). During 1998-2000, larval surveillance was carried out in 26 urban and rural areas in the Tohoku district, in the northern part of Honshu Island, by collecting larvae from artificial and natural habitats. Climatological analysis, using the GIS, showed that the following conditions accounted for the current distribution of Ae. albopictus: an annual mean temperature higher than 11 degrees C and a mean temperature of the coldest month, January, higher than -2 degrees C. A period with temperature above 11 degrees C in the confirmed area of the mosquito successively continues for more than 186 d per year. The accumulated temperature calculated from a temperature of 11 degrees C, which may be close to the developmental zero of Ae. albopictus, was over 1,350 degree-days. The relationship between the beginning of short-daylength, inducing egg diapause, and the monthly mean temperature during September and October necessary for successful larval development in the Tohoku district is also discussed. We also show the relationship between the current distribution of Ae. albopictus and the annual mean temperature in the United States. From these results it is predicted that Ae. albpictus will be established in some cities in northeast United States.