We investigated the spatial distribution and pyrethroid susceptibility of the mosquito larvae belonging to Aedes albopictus and Culex pipiens group in catch basins located in parks in Nagasaki city, Nagasaki, Japan. Among the 308 parks located in the central regions of the city, 194 were investigated. Cx. pipiens group larvae were collected from 31 sites; larvae of Ae. albopictus, from 34 sites. The Cx. pipiens group larvae were identified by PCR: 93.4% were found to belong to Cx. pipiens pallens, and 0.9%, to Cx. pipiens form molestus. A bioassay was performed by observing the knockdown of larvae during 30-min exposures to 0.4- and 0.1-ppm solutions of d-allethrin. High tolerance to d-allethrin (susceptibility index = 36) was observed in only 1 colony of Cx. pipiens pallens across 24 sites. On the other hand, Ae. albopictus showed high tolerance (susceptibility index > 30) in 8 of 22 sites; this indicated that Ae. albopictus populations tolerant to pyrethroids were spreading widely in Nagasaki city. The organized and massive larvicidal treatment of graveyard containers with DDT in the 1950s was thought to be one of the main causes for the development of pyrethroid resistance in Ae. albopictus.