The objective of this study was to investigate the performance of a pilot-scale membrane bioreactor (MBR) system for the treatment of a highly concentrated mixed wastewater from wet processes (dyeing, finishing, and sizing) of a denim producing textile industry. The MBR system, containing a Submerged hollow fiber membrane module in the aeration tank, was operated aerobically for about 3 months on-site at a continuous flow mode. The system was operated at two different operation stages: (1) 110 Sludge wastage with a typical permeate flux of 20 L/m(2)h and (2) a solids retention time of 25 days with the same flux. During the whole operation period, despite the high dissolved solids content in the influent, very high treatment efficiencies were achieved. The performance of the MBR system was not adversely affected by the variations in the influent characteristics, food/microorganism ratio, organic loading rate, and specific substrate utilization rate. The average values of some effluent quality parameters found In the entire operation period were color, 53 Pt Cc; turbidity, 0.31 NTU; TSS, 0.6 mg/L; BOD5, 15 mg/L; COD, 37 mg/L; NH3-N 1.0 mg/L; NO3-N, 9.6 and TN, 10.5 mg/L. Reduction in permeate flux due to membrane fouling was observed only once during all operation and such fouling was removed by applying chemical backwashing and chemical cleaning procedures. The results indicated that complex and highly polluted denim textile wastewaters could be treated very effectively by MBR systems. Color values from as high as 8100 Pt Co levels were significantly reduced to about 50 Pt Co levels, indicating that MBR effluent could be reused in the production processes.