In the course of investigations realized by us earlier it was found that there was no difference between radioresistance of microbes taken from various water sources. As a matter of fact quality of the microflora clearly reflects a unique phenomenon called selection which causes disappearance of all radiosensitive and survival of radioresistant types of microbes. There is indeed an increased number of radioresistant types of microbes with intensified activity of catalase and nuclease in pool water of atomic reactors.