The mosquitoes of Culex pipiens molestus first recorded from Leningrad as far as 30 years ago have been reported from the settlements of the Leningrad district since 1965. The most northern point of their occurrence is marked at 60 degrees 45' N and is removed from Leningrad for over 120 km. Such spread of mosquitoes to new settlements is associated with the development of multistory housing construction in the country and delivery of imagos from Leningrad to the settlements of the district by means of transport.