Abstract Several strict anaerobic bacteria belonging to different phylogenetic taxons were isolated from the Tatar and Siberian oilfields under different physico-chemical conditions. Data on their morphology and physiology were presented. All isolated strains were capable of producing oil-releasing compounds: biopolymers, organic acids or gases. Methanogenic bacteria were shown to produce polysaccharides. The polysaccharide of Methanococcoides euhalobius was partly purified and characterized. An active process of modern acetogenesis from CO2 + H2 was revealed in oilfields that were studied. Some acetogenic strains capable of producing volatile fatty acids from CO2 and H2 were isolated from stratal waters of the Tatar and Siberian oil fields.