The stability of biodegradation plasmids NPL-1 and NPL-41, which control the synthesis of enzymes for naphthalene oxidation to salicylate, was studied in Pseudomonas putida BSA under the conditions of its continuous cultivation with limitation in glucose or salicylate in the chemostat regime and without limitation in the pH-stat regime. Plasmid NPL-1, which controls the inducible synthesis of naphthalene oxygenase, is stable in the population of P. putida cells under the conditions of continuous cultivation on glucose, but is not stable in the course of cultivation on salicylate, an inductor of the naphthalene oxygenase synthesis. Plasmid NPL-41, which controls the constitutive synthesis of naphthalene oxygenase, is not stable in the population of P. putida cells under the conditions of continuous cultivation on glucose. The operation of genes, which control the oxidation of naphthalene to salicylate (nah), makes plasmids NPL-1 and NPL-41 unstable under the conditions of continuous cultivation in the absence of naphthalene from the medium, i.e. under the conditions when the expression of these genes is not necessary. In that case, cells containing plasmids with a deletion of nah-genes as well as cells without plasmids appear in the population of P. putida, which causes a decline in its futile energy and metabolic processes.