The potential for benzene, toluene, ethylbenzene, and xylenes (BTEX) degradation was investigated in microcosms inoculated with sediment and groundwater from a polluted iron-reducing aquifer. Benzene, toluene, and each of the three xylene isomers were degraded by the intrinsic microorganisms under iron-reducing conditions, but there was no removal of ethylbenzene. This work provides the first evidence for para-xylene degradation by dissimilatory iron-reducing bacteria. Microcosms adapted to benzene, toluene, or m-xylene were subsequently exposed to a different BTEX compound, which was degrade without lag phase, suggesting that the same group of bacteria could be involved in the removal of more than one BTEX compound. Furthermore, when microcosms were exposed to a mixture of BTEX, concurrent degradation of benzene and toluene, but not of meta-xylene and ethylbenzene, was observed. These results suggest that, under the influence of the plume of pollution, an iron-reducing microbial community able to degrade multiple aromatic compounds has developed.