The investigation was carried out in the “Seven Islands” nature reserve. It has been discovered that in the spring, summer and autumn the number of determined pollution degree indicator bacteria (TVC 22°C and TVC 37°C) of the water of Lake Oświn ranged from tens to thousands of CFU per 1 ml, depending on the research station. The sanitary state indicator bacteria (TC, FC, FS) occurred from single cells to several hundred thousand cells per 100 ml of water. In 36.5% of the lake water samples, the FC:FS ratio was deter-mined as higher than 4.0, but it also ranged from 0.7 to 4.0. In 18% of Lake Oświn water samples it ranged from 0.043 to 0.7, and in 9% it was lower than 0.043. This proves excretal contamination with prevailing excrements of human origin.