Rhodobacter sphaeroides AS1.1737 decolorized more than 90% of several azo dyes (200 mg dyes l(-1)) in 24 h. The optimal culture conditions were: anaerobic illumination (1990 lx), peptone as carbon source, temperature 35-40 degreesC and pH 7-8. Intracellular crude enzyme from this strain had azoreductase activity, optimized temperature as 45-50 degreesC, and decolorization kinetics which were consistent with a ping-pong mechanism.