Pseudomonas putida BS238 carrying the naphthalene degradative plasmid pBS2 was immobilized by adsorption on chromatographic paper and fixed on the surface of a Clark electrode. Naphthalene activated the respiratory activity of the cells, which was the basis for determining its concentration in one-component aqueous media. The lower limit of naphthalene detection was in the region of 0.9 mu M. The maximal responses of the sensor were observed at pH 6.0-6.2, 20-25 degrees C and a salt concentration 2.9-3.2 g ion/litre. The sensor responded weakly to the presence of biphenyl, monocyclic aromatic compounds, ethanol, carbohydrates and organic acids in the medium. A sensor based on an isogenic strain without plasmid pBS2 had no response to naphthalene. The possibility of improving the selectivity of naphthalene detection using a pair of sensors based on the original and isogenic strains is discussed.