Diclofenac is a non-steroidal anti-inflammatory drug (NSAID) that is used to treat the symptoms of pain and inflammation. Diclofenac and its metabolites enter to the ecosystem mainly due to the ineffective treatment in wastewater treatment plants and lead to toxic effects on the environmental ecology. Emerging contaminants such as diclofenac can be degraded to a large extent in anaerobic digestion unit of wastewater treatment plants. Thus, it is important to assess the fate of diclofenac under anaerobic conditions. Biodegradation potential of diclofenac under anaerobic conditions was investigated with a fermentative/methanogenic culture enriched from anaerobic digestion sludge of a municipal wastewater treatment plant. An anaerobic lab-scale semi-continuous reactor was operated at 35 degrees C in the dark. The culture was amended with glucose, yeast extract and diclofenac dissolved in methanol (10 mu g/L) providing 343 mg/L chemical oxygen demand (COD). A constant biogas production in the culture at the end of each feeding cycle was observed after 20 days incubation period (320 mL) which is in accordance with the theoretically calculated gas production. Although diclofenac removal was not achieved at the initial days of the acclimation period, after 70 days acclimation period partial diclofenac degradation (21%) was observed in the reactor. A batch experiment was conducted to assess diclofenac removal efficiency as well as the effect of diclofenac on methane and volatile fatty acids (VFAs) production for one feeding cycle. Although only 19% diclofenac removal was observed in the batch culture amended with 10 mu g/L of diclofenac, a negative effect was not observed in terms of gas production, VFAs production and consumption during the incubation period. The results indicate the importance of the further investigation to achieve complete degradation of the diclofenac under anaerobic conditions.