During 2015 in Dniester river the phytoplankton was represented by 67 species and varieties of planktonic algae from 6 taxonomic groups: Cyanophyta – 4, Bacillariophуta – 34, Euglenophyta – 4, Chlorophyta – 22, Chrysophyta-1 and Pyrrophyta – 2. The most frequent were the green and diatomean aldae, of which dominant were: Monoraphidium contortum, Scenedesmus quadricauda, Gomphonema olivaceum, Cocconeis placentula, Navicula cryptocephala, Nitzschia acicularis, Nitzschia sigmoidea. In medial sector of Nistru river during the investigations the values of phytoplankton number oscillated between 1,56-12,661 mln cel./l, and of biomass between 1,5046,919 g/m3. In spring period the number values in medium sector oscillated between 1,56 -5,29 mln cel./l, with the biomass from 2,24 to 6,24 g/m3, with the highest number in Naslavcea and Otaci stations. The values of phytoplankton number in summer period varied between 1,497-7,43 mln cel./l, with biomass of 1,967-6,064 g/m3. Hagher values were registered at Otaci and Soroca stations. In autumn period the values of number and biomass of planktonic algae were higher than in spring and summer, varying between 2,23-12,661 mln cel./l, with biomass of 0,95-6,919 g/m3, being higher at Soroca (fig.1). In this period, the high ratio of species from Cyanophyta group (8,933 mln cel./l) was observed: Oscillatoria lacustris and Synechocistys aquatilis. Fig. 1. Seasonal dynamics of number (N -mln cel./l) and biomass (B -g/m3) of phytoplankton in medium sector of Nistru river in 2015 In lower sectorul of Nistru river there increased values of phytoplankton number were registered– between 1,16-18,46 mln cel./l and of biomass between 1,11-9,17 g/m3. In vernal period the phytoplankton number oscillated between 1,16-7,82 mln cel./l with biomass 1,11-9,17 g/m3, being higher at Palanca station with the dominance of species from Cyanophyta (5,599 mln cel./l) and Bacillariophyta (8,372 g/m3) groups. In summer period the highest values of phytoplankton number (6,228-17,995 mln cel./l) and of biomass (2,114-6,173 g/m3) have been registered. At Vadul-lui-Voda (16,266 mln cel./l) and Varniţa (9,066 mln cel./l) station the increased number was caused by the intense development of species from Cyanophyta group: Oscillatoria lacustris and Synechocistys aquatilis. In autumn period the number varied from 6,03 to 18,46 mln cel./l, being higher at Palanca station caused by intense development of cyanophita algae (15,265 mln cel./l). The biomass of planktonic algae in autumn period registered lower values than in spring and summer periods. The biomass values oscillated between 1,308- 4,794 g/m3, being higher only at Palanca station (fig. 2.). Fig.2. Seasonal dynamics of number (N -mln cel./l) and biomass (B -g/m3) of phytoplankton in lower sector of Nistru river in 2015 A tendency of decreasing of phytoplankton biomass from spring to autumn was established. Across the river the maximal values of biomass did not exceed the limits of trophicity category “eutrophic” and lowest values fell within the category “mesotrophic”. Of the total number of algae species that have been identified (67 species and varieties) 43 are indicator species of water saprobity. During 2015 there was recorded the share of -mesosaprobe species, which constituted 60%, the most frequently encountered were: Cocconeis placentula, Cyclotella Kuetzingiana, Cymbella lanceolata, Diatoma vulgare v.vulgare. Analysis of saprobity index values in the medial and lower sectors of Nistru river allow us to find significant differences in the level of pollution of the river. In medial Nistru the saprobity index values ranged from 1.4 to 2.207 and in lower between 1.84 to 2.46 and fall within classes II and III of water quality (good-moderately polluted). Acknowledgement: The study was performed in the frame of institutional project 15.817.02.27A and bilateral Republic of Moldova-Belarus 15.820.18.02.06/B