Biological treatment of wastewaters containing textile dyes, particularly azo dyes, is very difficult under aerobic conditions. However, better decolourization of textile dyes can be obtained under anaerobic conditions. The azo bond (-N=N-), which is the chromophore group in the chemical structure of azo dyes, can be much more easily broken down under anaerobic conditions. This study aims at evaluating the anaerobic treatment potential of azo dye bearing textile wastewaters in terms oforganic matter removal, decolourization, and biogas generation. For this purpose, several 200 mL batch bioreactors containing anaerobic sludge, synthetic textile wastewater and azo dyes at various concentrations were incubated at 35 degrees C. The azo dyes used were Remazol Brilliant Violet 5R, Acid Orange 8, Naphtalene Blue Black and Remazol Black B. Significant biogas production was obtained in the presence of 10-500 mg/L of azo dyes. Hence, inhibitory effect of azo dyes on glucose biodegradation was limited even at high concentrations. Besides, decolourization efficiency reached almost 98% depending on the molecular structure of azo dyes. However, COD removal remained usually below 50% due to inhibition caused by aromatic amines, which are produced as a result of anaerobic degradation of azo dyes.