Systematic control of mosquitoes (C. p. molestus) has begun in Leningrad since 1968 when undertaken inspections revealed mosquitoes and their aquatic stages in 21.8% of basements. All hatching sites of mosquitoes were registered and sanitary, hydrotechnical and destroying measures were carried out there. During the period of 1968 to 1973 the basements were cleaned, in 509 of them hydroisolation was made and many were dried. Anually chemical treatment of basements was undertaken and for 6 years the treated area amounted to 6.00000 m2. As a result, the percentage of flooded basements decreased from 25.8 to 7.1% and the hatching sites of mosquitoes--from 21.8% to 9.1%. The total population of mosquitoes decreased 43 fold.