The blackflies on the south-western macroslope of the Ukrainian Carpathians are represented by 45 species. Forty of them are registered in the rivers (Simpsonís diversity index (≤s) equals to 0.019), and 29 species in the streams (≤s ó 0.090). Twenty-four simuliid species are common for both types of water ways (Czekanowski/Sorensen similarity index (≤) is 0.4). Only 16 species develop in the rivers, and 5 types of blackfl ies live only in the streams. Mass development of the following species in hydrobiocoenoses on the south-western macroslope of the Ukrainian Carpathians: Simulium trifasciatum and S. ornatum. Species such as S. ornatum, S. reptans and S. trifasciatum are widespread here. Simuliid faunas of main regional altitudinal plant groups differed in composition and quantitative correlation of species. Simulium argyreatum, S. intermedium, S. ornatum, S. trifasciatum and S. variegatum were eurytopic. The reachest species composition (29) was recorded in Rakhiv subdistrict in the district of spruce-fir-beech, spruce-beech-fir and spruce-beech Transcarpathian forests, and the poorest composition (15) is in the beech forests of southern megaslope of the Polonynian Beskyd, subdivision of beech Carpathian forests.