Specific growth rate, duration of the lag phase, stability of plasmids, and activities of the key enzymes involved in naphthalene biodegradation were studied in rhizospheric pseudomonades carrying structurally similar plasmids pOV17 and pBS216. It was demonstrated that these plasmids determined various levels of catechol 2,3-dioxygenase activities. The structural rearrangements in the plasmid pBS216 could "switch off" the genes of catechol oxidation meta-pathway. It was shown that certain combinations of biodegradation plasmids and bacterial hosts, such as Pseudomonas chlororaphis PCL1391(pBS216), P. chlororaphis PCL1391(pOV17), and P. putida 53a(pOV17), were considerably more efficient than natural variants in their growth characteristics and stability of the biodegradation activity, having a potential for bioremediation of soils polluted with polycyclic aromatic hydrocarbons (PAHs).