In the period 1993-1996, the Maltański Reservoir was found to host 233 phytoplankton taxa, belonging to 9 taxonomic classes. The most numerous were cyanobacteria. After regression of the Cyanobacteria-induced water blooming in the spring, chrysophytes, cryptophytes, green algae or diatoms were domi- nant. The remaining taxonomic groups were clearly less numerous. Changes in the number of a cells and of individual organisms and changes in the phytoplankton biomass were monitored in seasonal cycles. The shares of individual size fractions, i. e., of microplankton (>60 µm) and nanoplankton (2-60 µm) in the total numerical force and biomass of phytoplankton were estimated. Considering the size structure of phytoplankton organisms, nanoplankton comprised 50-100%, particularly in the period between November, 1994 and April, 1995, as well as in the early summer (May, June). Microplankton prevailed in total numbers of organisms between April and October, in particular. High correlation were noted between the number of organisms, number of cells and biomass of phytoplankton on one hand and the physico-chemical parameters of water such as transparency, temperature, pH, BOD5 index, conductivity and total phosphorus on the other. This pointed to the role played by these parameters in the development of algae. On the other hand, dissolved phosphates, nitrite and ammonium nitrogen exerted no limiting effects on the development of phytoplankton.