One hundred thirty eight taxa of plankton algae were identified in the Żur Reservoir (Cyanoprokaryota 27, Dinophyta 5, Euglenophyta 4, Chrysophyceae 6, Bacillariophyceae 40, Chlorophyta 56). The biomass of spring and summer plankton consisted mainly of diatoms. Based on the species composition, phytoplankton biomass, concentration of chlorophyll