Drastic threat to the natural system is caused by the uncontrolled release of synthetic pollutants, including azo dyes. This study centered on the decolorization and biodegradation of water soluble azo dye reactive blue (RB) in a batch mode sequential anaerobic-aerobic processes. A local sewage treatment plant was the source where activated sludge was collected to be used as non-adapted mixed culture with both free and the alginate immobilized cells for RB biodegradation. Under anaerobic conditions, the free and immobilized mixed cells were proved to completely decolorize 10 mg/ L of RB within 20 and 30 h, respectively. Alginate- immobilized mixed cells, resulted in 88%, 87%, and 87% maximum COD removals with samples containing RB at initial concentration of 10, 20, and 40 mg/L, respectively.  UV-vis spectra showed the biological cleavage of the azo bond in the anaerobic phase. Estimation of the phytotoxicity of the degraded metabolites suggested that the non-adapted immobilized mixed bacterial cells successfully detoxified RB azo dye.