The aim of the present study was to identify seasonal changes in net phytoplankton communities as indicators of the trophic state in two lakes with differing morphometry and trophic states. These studies were conducted in Lake Maróz from May to November 1995 and 1996 and in Lake Jeziorak Mały in 1996 and 1997. Differences in the abundance, biomass, species diversity, and dominant taxa of particular groups of phytoplankton between these lakes were analyzed. Lake Jeziorak Mały, which has a smaller area and shallower depth, is more susceptible to degradation than is Lake Maróz. The results of studies confirm that there are differences in the net phytoplankton communities and physiochemical water parameters in the studied lakes. The abundance of blue-greens (60760 indiv. dm-3), dominated by