In an earlier study, Pseudomonas putida NCIB 9816-4. which was one of the most studied naphthalene-degrading bacteria, showed the preferred adhesion to the naphthalene-contaminated soil when it was in the exponential growth phase. The adhesion was found to take place through a hydrophobic interaction. We postulated that the surface hydrophobicity of P. putida NCIB 9816-4 in the exponential growth phase might be increased during the uptake of naphthalene, which caused the preferred adhesion to the naphthalene-contaminated soil. To verify this postulate. a plasmid-cured strain of P putida NCIB 9816-4 was obtained in this study and compared with the wild-type for adhesion to the naphthalene-contaminated soil. Only the wild-type in the exponential growth phase showed increased adhesion to naphthalene-contaminated soil. The water contact angles of the two strains were measured in the presence and in the absence of naphthalene as indices of surface hydrophobicity. The water contact angle of the wild-type increased in the presence of naphthalene, whereas that of the cured strain did not change. We conclude that the uptake of naphthalene during naphthalene biodegradation in the exponential growth phase of P putida NCIB 9816-4 made the cell surface more hydrophobic, resulting in increased adhesion to naphthalene-contaminated soil. (C) 2009 Elsevier B.V. All rights reserved.