A model system was developed for evaluating naphthalene biosorption based on the use of a mutant (strain TG-5 Nah(-)) derived from a naphthalene-degrading Pseudomonas fluorescens isolate. Cells of strain TG-5 had a sorptive capacity for naphthalene (partition coefficient of 380 cm(3)/g) significantly higher than a soil with a 5.1% organic carbon content (partition coefficient of 41 cm(3)/g). However, experimental results and a mass balance model demonstrated that, in soil systems of high organic carbon content, the mass of naphthalene associated with biological solids is insignificant. In contrast, in a soil system of nonsorptive Ottawa sand, up to 10% of the initial naphthalene was demonstrated experimentally, and by modeling, to be associated with cells of strain TG-5.