The studies on the changes in phytoplankton composition against a background of selected physico-chemical factors of water in Lake Laskownickie (near Wagrowiec, Poland) were carried out in spring-summer seasons of 1993 to 1995. The changes in the qualitative and quantitative composition of phytoplankton and dynamics of Cyanoprocaryota populations in the vegetation season were connected with high concentrations of the phytoplankton biomass (10-70 mg dm^3), chlorophyll a (18.7-195.7mug dm^3) and biogens (Tot. P 0.040-1.100 mg P dm^3; Tot. N 3.77-12.00 mg dm^3). The blooms of Cyanoprocaryota occured in this reservoir each year. They were dominated by species such as: Plankto-thrix agardhii (Gom.) Anagnostidis et Komarek, Limnothrix redekei (Van Goor) Meffert, Aphanizomenon issatschenkoi (Vsac) Prosk-Lavr, Pseudoanabaena limnetica (Lemm.) Kom.. During blooms, the changes of the composition of companion taxa were observed.