The results of river water monitoring at the licensed oil fields in the Middle Ob' area are presented. A complex estimation of the river water ecological conditions has been made using microbiological indication and biotesting. The number of heterotrophic saprophytic bacteria in the majority of rivers corresponds to a satisfactory water quality. Fecal pollution was detected in most rivers. The self-purification potential of the major part of streams was low. Gram-positive coccus bacterial forms were dominant in the microbal cenoses in the majority of rivers. Seven toxic samples were detected as a result of biotesting.