The article presents the results of the investigations on diversity and quantitative parameters of planktonic algae in three rivers: Raut, Bâc and Botna, which are the tributaries of the Dniester River. In tributaries of Dniester River phytoplankton was represented by the four groups: Cyanophyta, Bacillariophyta, Euglenophyta and Chlorophyta. It was established that the biggest negative effect on the Dniester River influences the water coming from the Bâc River according to the quantitative parameters (number of species and biomass) of phytoplankton as well as the saprobity index of planktonic algae. The values of the saprobic index of this tributary fell within the limits of water quality classes III (polluted). The water quality of the Răut and Botna tributaries according to the values of the saprobic index, being assigned to class II (moderately polluted).
