Polycyclic aromatic heterocycles, such as carbazole, are environmental contaminants suspected of posinghuman health risks. In this study, we investigated the degradation of carbazole by immobilized Sphingomonassp. strain XLDN2-5 cells. Four kinds of polymers were evaluated as immobilization supports for Sphingomonassp. strain XLDN2-5. After comparison with agar, alginate, and carrageenan, gellan gum was selected as theoptimal immobilization support. Furthermore, Fe3O4 nanoparticles were prepared by a coprecipitation method, andthe average particle size was about 20 nm with 49.65-electromagnetic-unit (emu) g-1 saturation magnetization.When the mixture of gellan gel and the Fe3O4 nanoparticles served as an immobilization support, the magneticallyimmobilized cells were prepared by an ionotropic method. The biodegradation experiments were carriedout by employing free cells, nonmagnetically immobilized cells, and magnetically immobilized cells in aqueousphase. The results showed that the magnetically immobilized cells presented higher carbazole biodegradationactivity than nonmagnetically immobilized cells and free cells. The highest biodegradation activity was obtainedwhen the concentration of Fe3O4 nanoparticles was 9 mg·ml-1 and the saturation magnetization ofmagnetically immobilized cells was 11.08 emu g-1. Additionally, the recycling experiments demonstrated thatthe degradation activity of magnetically immobilized cells increased gradually during the eight recycles. Theseresults support developing efficient biocatalysts using magnetically immobilized cells and provide a promisingtechnique for improving biocatalysts used in the biodegradation of not only carbazole, but also other hazardousorganic compounds.