The biodegradation of naphthalene in the presence of Na montmorillonite particles, of montmorillonite/polyacrylamide complexes and of polyacrylamide alone was studied in batch assays under aerobic conditions. The concentrations of naphthalene used were below aqueous phase saturation. Adsorption-desorption experiments with montmorillonite and montmorillonite/polyacrylamides complex indicated that little or no naphthalene was adsorbed. Adsorption appeared totally reversible and desorption was rapid, reaching completion in less than 24 hours. Naphthalene was rapidly degraded by an adapted strain of Pseudomonas cepacia (10-25 hours) as a function of the initial viable bacteria count of the inoculum. The kinetics of naphthalene degradation in the presence of montmorillonite/polyacrylamide complexes or even with montmorillonite alone (with no naphthalene adsorbed), were slower (0.64 mg l−1 h−1) than in clayless aqueous solution (0.84 mg l−1 h−1). Dissolved non-biodegradable polyacrylamide had no effect on naphthalene degradation. These results indicate that particles decrease the rate of naphthalene breakdown byPseudomonas cepacia . This may be caused by the fact that montmorillonite particles are smaller on average than bacteria cells. Montmorillonite particles may cover bacteria and so limits nutrient mass transfer.