This report presents results of four-year studies of bacterial production and biomass, and selected environmental variables (concentrations of total DOC, microbiologically labile DOC, chlorophyll^a) in surface pelagic waters of four Mazurian lakes of differing trophic status (oligo/mesotrophic, eutrophic, hypereutrophic, polihumic) during summer stratification periods 1994-97. Bacterial production and biomass were positively proportional to the degree of lake water eutrophication. The rates of production of bacteria and their biomass turnover were primarily dependent on concentrations of microbiologically labile organic substrates in the DOC pool. In lakes with high content of suspended particulate detritus (hypereutrophic and polihumic lakes) attached bacteria significantly predominated in total bacterial production. Importance of the "bottom-up" and "top-down" mechanisms in ecological regulation of bacterial production and biomass in the studied lakes is widely discussed.