The results of special tests which had been carried out with the following bacterail strains Bacillus sp. S2, Bacillus sp. BP, Pseudomonas sp. 141 and Micrococcus sp. R5.4 show that these strains are acive degraders of petroleum oil hydrocarbons, tolerate high concentrations of the subsrtate and have a high capability for degradation of hydrocarbons.The activity of dehydrogenases was related to the concentration of the substrate.The gas chromatography analysis of residual hydrocarbons in the medium after 9 days of cultivation proved that the main identified petroleum oil components had been metabolized with the yield 69 to 100%. Apart from the compounds originally present in crude oil no new by-products were detected in final medium.The accumulation of shorter chain length hydrocarbons and various new by-products occured in the case of another tested bacterial strain, designated 'B'.