Abstract
It was demonstrated that the number of saprophytic fungi in the population of plankton in mesotrophic lake changing to eutrophic (Lake Piaseczno) was mountained a similiar level (average values) as in the eutrophic one (Lake Głębokie). The seasonal and annual changes in the number of fungi in the waters of the lake with lower trophicity were markedly stronger than those in the lake with higher trophicity. In the mesotrophic lake this was connected with the intensity of phytoplankton development. The greatest accumulation of fungi occured in the waters of littoral zone in both lakes and in the pelagial metha- and hypolimniun of the Lake Piaseczno.