Combination of adsorption and biodegradation is becoming more common to remove phenolic compounds from wastewater. Adsorption could reduce the inhibitory effect of the compounds for microorganisms, whereas biodegradation could, in some extend, freeing adsorption sites. This results in extending the life of adsorbent. Activated carbon is the adsorbent widely used for that purpose. This study deals with the use of Indonesian natural zeolite to remove phenol in a laboratory scale fixed bed system, in the presence of microbial growth. 50 mg/l of phenol solution was fed into the bed at the flow rate of 1 cm(3)/s. The bed was inoculated with Psudomonas putida. It was observed that the microbial mass has slowed down the adsorption, as it contributed in increasing mass transfer resistance. The effect was more significant at higher cycles. About 2% of adsorbed phenol was microbially degraded into carbon dioxide, whereas the rest could be converted into dissolved products and microbial mass, and/or irreversibly adsorbed. Comparison between the observed data with predicted data indicates that the system was much more complex than merely combination of adsorption and biodegradation processes. Further evaluation should be made. (C) 2003 Elsevier B.V. All rights reserved.