Waters of the Truskavetz deposit with a low content of mineral substances were found to contain considerable amounts of microorganisms; upto millions of cells per 1 ml. Sulphate-reducing, dentrifying and iron bacteria were detected in the waters. The following genera were represented and isolated: Pseudomonas, Bacillus, Nocardia, Arthrobacter, Brevibacterium, Corynebacterium, Micrococcus, Candida, Cryptococcus, Rhodotorula, Hansenula, Torulopsis, Trichosporon. Many of the studied strains assimilated C6--C10 and C12--C22 mixtures, industrial paraffins, naphthenic acids and their salts. The isolated bacterial and yeast strains were much more carbooligotrophic as compared to the museum strains of the same species; many of them were oligotrophic. The cells continued to grow in the course of storage of isolated water samples. The majority of isolated pure cultures grew upon inoculation into sterilized mineral water and caused a decrease in the content of organic substances.