The aim of this study was to determine the biomass and abundance of fungi in the waters of the shallow and polyhumic reservoir located in northeastern Poland. It was shown that fungal biomass in water positively correlated with abundance (CFU/ml, colony-forming unit/ml) and negatively with the number of morphotypes. The largest biomass of fungi was recorded in October (0.55 μg/dm³) and the lowest in winter (0.22-0.28 μg/dm³), when fungal colonies had greater phenotypic diversity. The main river supplying the reservoir had greater average fungal biomass and abundance than the reservoir.