The chemical composition and toxicity for living organisms-aquatic organisms of reservoir (waste) and surface (for injection into reservoir) waters of production facilities of the LUKOIL-Ukhtaneftegaz RPC were studied. It has been established that highly mineralized formation waters have acute toxicity for daphnia, green unicellular algae and fish fry, which is eliminated by diluting the samples 2–100 times. Indicators of the Lethal Dilution Ratio of LDR50 and lethal concentrations of LC50 of the studied water samples were calculated for test organisms, which ranged from 10,780 to 27,910 mg/l, averaging 21,140 mg/l. It was concluded that the studied samples of formation water, which differ significantly (by 3–4 orders of magnitude) in a positive direction from the standard LC50 concentration of 10 mg/l, cannot be classified as substances that pose a hazard to the natural environment.