In this study, anaerobic treatment of synthetic textile wastewater containing a reactive azo dye, namely, Remazol Brilliant Violet 5R, was investigated. A fluidized bed reactor (FBR) was used in the study. Before the operation period, start-up of the FBR was completed in 128 days with an immobilized microorganism level of 0.069 g volatile suspended solids per g support material (pumice). Anaerobic treatment of synthetic textile wastewater revealed that 300 mg/L dye was removed in the FBR system. Chemical oxygen demand (COD) and color reduction in the system were approximately 60 and 94%, respectively. Under anaerobic conditions, formation of two sulfonated aromatic amines (SAAs) was detected due to anaerobic reduction of the dye. The SAAs were not degraded under anaerobic conditions. In addition to the anaerobic treatment, the effectiveness of aerobic treatment was investigated in order to further reduce the COD after the anaerobic treatment.