The world fauna of black flies contains more than 1000 species classified into 59 genera (Rubzov, 1974). Four hundred species referable to 25 genera are found in the Soviet Union. These are small and morphologically uniform flies, therefore the species diagnostics is very difficult. Valuable information may be derived from the analysis of polytene chromosomes. We have applied this method to the family Simuliidae. During the last few years we have analyzed the chromosomes of about 100 species from different genera, collected in various geographic regions of the USSR, using the aceto-orcein squash technique. Intraspecific, intrageneric and intergeneric analysis were carried out (Chubareva, 1974, 1977a, b, c, d, 1978a, b) in which the main chomosome markers, banding sequences and types of rearrangements were compared. From the available data one can conclude that the Simuliidae is karyologically a very homogeneous group. The majority of species studied have 2n = 6; the only species found so far with 2n = 4 are Cnephia lapponica (Petrova, 1972) and members of the genus Eusimulium ('aureum group', Dunbar, 1958; Simonenko, 1966; Chubareva & Shcherbakov, 1963). This pattern shows that the chromosome number alone is insufficient to characterize the taxonomic affiliations and that much attention in karyological analysis should be given to structural peculiarities of polytene chromosomes. The diploid number of 4, characterizing all Eusimulium species, separates this from other genera of the family, and has attracted special attention of cytogeneticists (Dunbar, 1968, 1959; Chubareva & Shcherbakov, 1963; Simonenko, 1966). It should be mentioned that at this stage species of the genus Eusimulium are morphologically distinguishable neither in the adult nor in the larval stage, hence the karyological analysis greatly facilitates taxonomic diagnostics. This paper deals with karyotypic characteristics of the species in the genus Eusimulium Roub., and the position of the latter in the family. FormedyEusimulium was considered a large genus consisting of 5 species groups (montiun~ alpinum, latipes, angustitarse and aureum). Recently, due to research of Crosskey (1969) and Rubzov (1974), it was reclassified into four distinct genera, the old generic name being retained for the species of the aureum group in the fauna of the USSR. The six genera have been placed in the tribe Eusimuliini by Rubzov (1974). All of these have been studied karyologically by us (Table i).