Biodesulfurization of Mengen lignite by a mesophilic bacterium, Rhodococcus rhodochrous ATCC 53968, was investigated in a batch-stirred and aerated reactor. Experiments were carried out at 28degreesC with the inoculum percentage, initial pH, and initial sodium acetate concentration of the biodesulfurization medium as 8% (v/v), 6.5, and 20 mM, respectively. The effects of the lignite concentration and retreatment on the removal of different sulfur forms from coal were investigated. The highest decreases in the total and organic sulfur contents were obtained as 21.4% and 9.2%, respectively, at the operational conditions of the lignite concentration of 5 g/L at 500 rpm stirring and 0.18 vvm aeration rates. The total and organic sulfur contents of the coal after biodesulfurization were correlated with the coal concentration by using the linear least-squares regression method. In the rebiodesulfurization run that lasted 8 days, the highest organic sulfur-reducing percentage was obtained as 10.2% at the same experimental conditions.