The extensive nature of natural resource use in the Siret River basin in recent years has led to a significant depletion of its water resources and deterioration of the ecological state of surface waters. The following paper presents the results of assessing the degree of water pollution of the Siret River. A number of biotic water quality indices were used for this purpose. In particular, the Trent Biotic Index (TBI) and the Biological Monitoring Working Party Index (BMWP), both of which use the density ratio of different macroinvertebrate taxa in the aquatic communities, and the Pantlé and Bucca saprobility index, which uses the absolute density and saprobility of indicator species of aquatic organisms. A comprehensive assessment of the aquatic ecosystem of the watercourse was carried out using the water pollution index (WPI). The biological indices were calculated on the basis of data on zooperiphyton communities formed on an artificial fibre media of the «VIYA» type. The «VIYA» fibre carrier was used for more accurate determination of the species composition and quantitative characteristics of zooperiphyton communities in the studied river sections. The TBI index ranged from 5 to 6 (which corresponds to a water quality assessment of «satisfactory»). The BMWP index ranged from 8 (very poor) to 32 (good), and the saprobicity index from 1.52 to 1.95 (good water quality). The WPI index at the control sites in the Siret River ranged from 4.6 (which corresponds to a water quality assessment of «polluted») to 8.1 (which corresponds to a water quality assessment of «extremely polluted»). Significant differences in the results of the assessment by different biotic indices indicate that these indices cannot be used as indicators for an exhaustive assessment of water quality using zooperiphyton. They can only be used as relative indicators for comparing the state of water pollution at different locations. The values of hydrochemical indicators of water quality at two observation posts of the river were analysed. The maximum permissible concentrations of nitrite ions, ammonium ions and suspended solids in the control sections of the river were found to be exceeded.