Phytoplankton of Sulejow Reservoir was investigated in 1979-1993. Over that period, blue-green algae were usually a constant component of phytoplankton. Their percentage in the total phytoplankton biomass widely varied both from season to season and from year to year. Maximum values were noted in summer, although sometimes a shift of peak development toward spring or even winter was observed. The development was not uniform in the whole area of the reservoir; blue-green algae dominated in the middle and downstream, that is in the deepest parts of the reservoir. The main components of phytoplankton included Microcystis aeruginosa K?tz. and Aphanizomenon flos-aquae (L.) Ralfs.