Analysis of the species composition of the blood-sucking flies in different landscapes of St. Petersburg and Leningrad Province (the Region) was performed with the use of a specially developed database (DB), combining material of the collection of the Zoological Institute of the Russian Academy of Sciences, published data, and results of the recent studies (Aibulatov, 2009). In all, 165 species from 30 genera of blood-sucking mosquitoes, biting midges, blackflies, and horseflies are known from 255 localities in the Region. The gnus fauna of the eastern territories of Leningrad Province is still poorly known. The set of the commonest species is determined. The territory of the Region is subdivided into 10 subregions based on the parameters of the local climate, water supply, and natural drainage. The gnus fauna in each subregion is briefly characterized. The classification of the hydrolandscapes proposed earlier for Novgorod Province (Medvedev and Panyukova, 2005; Panyukova and Medvedev, 2008) is only partly applicable to the Region which lacks large lakes and rivers with floodlands where small water bodies providing places for larval development of bloodsucking insects can be formed. Constant population densities of certain species of the gnus complex are favored by underground water supply of the large rivers in the Region, whose tributaries run from the bogs and transitional mires in the interfluves.