Experiments on the survival of Escherichia coli were conducted in the water of man-made Zegrzyński Reservoir. The aim of this study was to evaluate how selected biocenotic factors (predation, coliphage infection, presence of autochthonic microflora and nutrient conditions) affect this process. We observed residual living cells of E. coli up to about thirty days of the investigation. We found that the major factor that was responsible for mortality of E. coli was microflagellate grazing and the exposure to aquatic environment. Size-selective preferences of microflagellates toward E. coli cells were observed. No visible effect of bacteriophages on the survival of E. coli was detected. Better nutritional condition and the presence of native heterotrophic microflora significantly prolonged the survival time of E. coli in the studied environment.