The effect of arsenite on the degradation of naphthalene, a polycyclic aromatic hydrocarbon (PAH), by P. putida strains BS202 (NPL-1) and BS238 (pBS2; pBS3031) was studied. Their resistance to arsenite is determined by either chromosomal or plasmid genes. In a soil model system supplied with arsenite (400 mug/g dry soil), the cell density of strain BS238 (pBS2; pBS3031), grown on naphthalene, remained virtually unaltered though naphthalene consumption was slightly delayed (1 day lag); naphthalene disappeared by the 6th day. Under the same experimental conditions, strain BS202 (NPL-1) was characterized by a long lag-phase during the first 2 days of cultivation and a slow consumption of naphthalene: by the 6th day naphthalene residues were still detectable in the soil model system. The effect of arsenite on naphthalene degradation by the PAH-degrading P. putida strains exhibiting different mechanisms of the arsenite resistance is discussed. (C) 2003 Elsevier Ltd. All rights reserved.