Numbers of different bacterial groups and physico-chemical water quality parameters were studied during four year period in two rivers receiving mine water from underground oil shale mine. Input of mine water causes increase of alkalinity, sulfate and calcium ion concentration in river water. Bacterial community structure of studied rivers was altered by mine water and had clear seasonal pattern. The results indicate that input of mine water increases number of viable heterotrophs and lipolytic bacteria in river water, especially during underground oil shale burning.