Phytoplankton composition and biomass were studied in a small suburban lake within the city of Poznań (western Poland) on the basis of water samples taken in epilimnion. Results obtained in 1999 were compared with the first study period, of 1978. In the first period, 100 phytoplankton taxa were found and diatoms were the largest group of species (29% of all taxa), while in the second period 117 phytoplankton taxa were recorded and green algae dominated (50%). The mean phytoplankton biomass increased nearly 2-fold over the two decades and the trophic state index was higher in 1999. Picophytoplankton, whose seasonal fluctuations were analyzed only in 1999, accounted for 0.6% to 31.5% (mean 12.8%) of total phytoplankton biomass.