<b>Abstract</b>— <p>Paleozoic siliceous–ferruginous rocks in massive sulfide deposits of the Urals contain well-preserved textural and structural evidence indicating the role of microorganisms in their formation. Filamentous (morphologically dominant), rod-shaped, spherical, and tubular micrometer-scale bacterial bodies with the hematite, hematite–silica, and siliceous mineralization are described. Authigenic apatite, Mn-calcite, Ti oxides, illite and rare earth minerals (phosphates and carbonates) are associated constantly with bacteriomorphic structures. Identification of structures of fossil bacteria based on their size, shape, cell division, and distribution in colonies and comparison with ferruginous sediments in modern hydrothermal systems and massive sulfide deposits in other world regions suggests that they correspond to mineralized stalks of iron bacteria Leptothrix ochracea and Gallionella ferruginea, coccoidal forms and sulfur-oxidizing Thiobacillus Ferrooxidans and giant Beggiatoa-type organisms. These observations indicate bacterial biocatalysis in the processes of halmyrolysis of sulfides and hyaloclasts during the formation of iron oxide sedimentary rocks.</p>