A bacterial strain, able to efficiently degrade benzene, toluene, ethyl benzene, and o-xylene ( BTEX) compounds, was isolated by acclimating and enriching the activated sludge from the aeration tank in refinery wastewater treatment plant using BTEX as the sole carbon source. Based on the morphological characteristics, physiological and biochemical characteristics, sequence analysis of 16S rDNA,and Biolog identification system,the isolate was identified as Mycobacterium cosmeticum which was a newly discovered species able to degrade BTEX. The optimal conditions for the growth of the strain were at 30 degrees C and pH 7.0. The order of BTEX degradation by this isolate is benzene,toluene, ethyl benzene,and o-xylene. The specific oxygen utilization rates (SOUR) of the strain degrading benzene,toluene, ethyl benzene, and o-xylene were 165.3, 170.5, 49.3 and 57.4 mg x (min x mg)(-1), respectively. The degrading process of the strain followed the Haldane kinetic model. The maximum specific degradation rate degrading benzene, toluene, ethyl benzene,and o-xylene were 0.518, 0.491, 0.443 and 0.422 h(-1), respectively. Accordingly,the maximum specific growth rate 0.352, 0.278, 0.172 and 0.136 h(-1), respectively.