Research on phytoplankton was carried out during 2019, samples were collected from lower sector of Prut River. Sampling and analysis of phytoplankton was carried out by conventional hydrobiological methods [1,2]. During 2019 year the diversity of quantitative parameters of phytoplankton, as well as the water quality, using algae species as indicators of saprobity were assessed. In phytoplankton compositions of the lower sector of Prut River were identified 42 species. It was found that the phytoplankton species of the following groups predominated: Cyanophyta, Bacillariophyta and Chlorophyta. The basis of the floristic diversity of Prut River consists of species: Aphanizomenon flos-aquae, Oscillatoria lacustris from green algae, Cyclotella kuetzingiana, Navicula cryptocephala, Synedra acus, Synedra ulna from diatoms algae, Monoraphidium Komarkovae, Monoraphidium contortum and Scenedesmus quadricauda from green algae. The number of phytoplankton species in Prut River was in limits 1.06-15.9 mln. cell/l. In lower sector of Prut River was attested the preponderance of Cyanophyta in the forming of the number of algae species. The biomass of planktonic algae was in limits 1.4-9.6 g/m3. The main part in the formation of phytoplankton biomass was Bacillariophyta algae: Cymatopleura solea, Gyrosigma acuminatum, Nitzschia sigmoidea, Surirella robusta, Stauroneis anceps. The saprobic index values were established within 1.93-2.32 at Prut river lower sector. The values of saprobic indices were estimated on the basis of species-indicators (29 species) from phytoplankton composition, which were in proportion of 62 % typically mezosaprobic. According to these indices, the water quality of Prut River lower sector was satisfactory for the development of phytoplankton and was attributed mainly to II-III (good-moderately polluted) quality classes.