Aeromonas sp. B-5, which has the ability to decolorize azo dyes, was isolated from soil. Aeromonas sp. B-5 completely decolorized 100 mg/l of Bordeaux S by reductive cleavage of azo bonds under static conditions in 24 h. Though the decolorization of Bordeaux S by Aeromonas sp. B-5 was suppressed under shaking, rapid decolorization was observed when the culture was changed to static conditions after cultivation with shaking. The indigoid dye, Acid blue 74, was decolorized by Aeromonas sp. B-5 under shaking conditions, in contrast to the decolorization of azo dyes.