Survival and development of larvae of Aedes aegypti (L.) and Aedes albopictus (Skuse) were examined under field conditions on the campus of Nagasaki University School of Medicine, Nagasaki, Japan, to explore the possibility of the establishment of Ae. aegypti in Kyusyu, southern Japan. Exposure experiments with hatched larvae were conducted 11 times at about 1-mo intervals from January 1998 to January 1999. In both species, larvae could develop to adults under field conditions from April to November, but not in January, February, and December 1998. In March 1998, only Ae. albopictus could develop to adults. The results suggest that there are differences in the resistance of hatched larvae to low temperatures between Ae. aegypti and Ae. albopictus and a threshold air temperature of larval development below 10°C for both species. To estimate the threshold temperatures of larval development, the relationships between the average air temperature and the developmental rate observed in March–November 1998 were analyzed by linear regression analysis. The estimated threshold air temperatures for Ae. aegypti were 8.52 and 9.45°C for females and males, respectively, and were higher than those of Ae. albopictus. A clear seasonal change was also observed in the body size of pupae. In both species, body size was largest at the beginning of the breeding season and continuously became gradually smaller until July. The results were compared with those from previous field studies in Nagasaki and the possibility of the establishment of Ae. aegypti in Nagasaki, Japan, is discussed from the viewpoint of adaptive life history to seasonally changing environments.