The physicochemical surface characteristics of a Tn-5 induced hydrophobic mutant (CE3003) ofRhizobium etliCE3 were investigated. The wild type CE3 was very hydrophilic with low contact angles for polar liquids, while the Tn-5 induced mutant had a surface that was moderately hydrophobic, with polar liquid contact angles in the 50–60° range. As a result, the polar surface free energy components (γ+and γ-) that constitute the acid-base component (γAB) of surface tension, were greatly reduced on THE surface of the hydrophobic mutant. This decreased electron donicity of the mutant' surface caused an almost five fold increase in the magnitude of the acid-base component of the interfacial interaction free energy between the mutant and hexadecane. The increased adhesion to hexadecane reported earlier is probably attributable to this interaction free energy change and not to any alteration in zeta potential, which was similar for CE3 and CE3003 at pH 7. X-ray photoelectron spectroscopy showed CE3OO3 to have less surface carbon and nitrogen and more surface oxygen than CE3 with alterations in the (C-C,H) and (C-O,N) components being observed.