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Bacterial biomass collected from sheath-forming bacteria of the genera Sphaerotilus and Leptothrix was collected from a high-mountain natural stream water source. The elemental constitution and oxide phases of the products after selective cultivation of the bacteria on two different elective media using neutron activation analysis (NAA), electron microscopy (SEM, TEM), and X-ray diffraction (XRD) were studied. A high enrichment level of iron was revealed by the NAA technique in cultivated isolates as compared to the reference sample from nature. Three types of iron oxide compounds were established after cultivation in Adler's medium: lepidocrocite (γ-FeOOH), magnetite (Fe3O4), and goethite (α-FeOOH). The cultivation in the Isolation medium yielded a single phase, that of goethite, excluding one sample with a distinguishable amount of lepidocrocite. XRD and EM investigations show that the biogenic oxides are nanosized. Our study exemplifies the possibilities of the biotechnology approach for obtaining, under artificial conditions, large quantities of iron-containing by-products that could be of further used in appropriate nano- and biotechnologies.