The stability of recombinant plasmid carrying genes for naphthalene mineralization was determined. A strain of Pseudomonas putida capable of mineralizing naphthalene (Nap+) via salicylate (Sal+) was isolated, and all regulatory and structural genes for the whole pathway were found to be encoded on a 25 kb EcoRI fragment of an approximately 83 kb plasmid present in this strain. The 25 kb EcoRI fragment was cloned into a tetracycline-resistant (TcR) cloning vector pLAFR3 and the recombinant plasmid, pRKJ3 (Nap+, Sal+, TcR), thus obtained was transferred into the plasmid-free strain Pseudomonas putida KT2442 in order to test the stability of the plasmid. Plasmid pRKJ3 was found to be segregationally and/or structurally unstable, depending on the growth conditions. Two types of novel derivative strains having the phenotypes Nap−, Sal+, TcR and Nap−, Sal−, TcR with specific deletions of approximately 2 kb and 18 kb, respectively, were obtained.