The Irtysh River is a transboundary watercourse that exhibits a wide range of climatic conditions, environments and ecological niches. As a result of the technogenic impact of industrial enterprises on surface and underground waters all the main tributaries of the middle reaches of the river Irtysh are in different degrees of pollution. Due to the impact of sewage and other wastes from the mining and metallurgical industry, in the waters of the river basin Irtysh the concentration excess of a number of toxic metals is observed. Among the invertebrates used to assess the state of natural waters, representatives of the Annelida type are often used. Since annelids are among the most common organisms on Earth, this makes it possible to study them in many regions and to assess the specificity of the impact and compare the impact of various pollutants in similar natural and climatic zones. It is known that the distribution of freshwater leeches is largely determined by natural and climatic conditions, landscape characteristics and the specifics of trophic connections, etc. In this case, the most important parameters are the physicochemical properties of water. This paper provides information on the factors influencing the species composition and structural indicators of leech communities living in the middle reaches of the Irtysh River.