Our studies were aimed at bacteriological analysis of water in the small municipal lake of Rusałka, in Szczecin. The studies were conducted over four years, in monthly intervals, assuming that the results will enable detailed analysis of bacteriological characteristics of the water in the tested reservoir and will yield sanitary and ecological characteristics of the reservoir. The quantitative estimation included index bacteria for the extent of pollution, index bacteria of sanitary condition, bacteria of selected physiological groups (ammonification bacteria, denitrification bacteria) as well as the presence of thermophile bacteria, sporulating bacteria, bacteria capable of sulphate reduction (Desulfotomaculum nigrificans). Also, the ratio was calculated for foecal-type coli group bacteria to foecal streptococci, treated as an index of origin of foecal-type pollution. The obtained results were subjected to statistical analysis. In the studied lake high values of all the examined parameters were disclosed. The high numerical force of index bacteria for the extent of pollution (TVC 20°C, TVC 37°C) and of index bacteria for sanitary condition (coli group bacteria, foecal type coli group bacteria, foecal streptococci) pointed to contamination of the lake by household sewage. The observed high numbers of bacteria participating in turnover of nitrogen compounds (ammonification and denitrification bacteria) indicated a continuous inflow of nitrogen-rich compounds and development of water self-purification. The values of most studied parameters obtained by us were in general higher than the values noted in the same lake in previous years and higher than the values noted in other municipal lakes in Poland.