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An efficient nitrobenzene degrading bacterium strain NB5, which was able to utilize nitrobenzene as a sole source of carbon, nitrogen and energy under aerobic condition, was isolated from activated sludge in an oil refinery at Hangzhou, China. Based on phenotypic features, 16S rDNA gene sequencing and G + C content analysis, strain NB5 was identified as Rhodococcus sp. NB5. Nitrobenzene degradation experiments using high performance liquid chromatograph (HPLC) showed that strain NB5 could tolerate a high nitrobenzene concentration and completely degrade nitrobenzene with initial concentration ranging from 100 mg · l(-1) to 1000 mg · l(-1) within 144 h. The optimal degradation and cell growth were observed at 30 °C, pH 7.0. The addition of second nitrogen source (0.1%) such as urea, peptone, yeast extract and beef extract generally enhanced degradation of nitrobenzene. Rhodococcus sp. strain NB5 could be an excellent candidate for biotreatment of industrial wastewater containing high concentration of nitrobenzene.