A sulfate-reducing consortium maintained for several years in the laboratory with m-xylene as sole source of carbon and energy was characterized by terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of PCR-amplified 16S rRNA genes and stable isotope probing of proteins (Protein-SIP). During growth upon m-xylene or methyl-labeled m-xylene (1,3-dimethyl-¹³C₂-benzene), a phylotype affiliated to the family Desulfobacteriaceae became most abundant. A second dominant phylotype was affiliated to the phylum Epsilonproteobacteria. In cultures grown with methyl-labeled m-xylene, 331 proteins were identified by LC-MS/MS analysis. These proteins were either not ¹³C-labeled (23%) or showed a ¹³C-incorporation of 19–22 atom% ¹³C (77%), the latter demonstrating that methyl groups of m-xylene were assimilated. ¹³C-labeled proteins were involved in anaerobic m-xylene biodegradation, in sulfate reduction, in the Wood-Ljungdahl-pathway, and in general housekeeping functions. Thirty-eight percent of the labeled proteins were affiliated to Deltaproteobacteria. Probably due to a lack of sequence data from Epsilonproteobacteria, only 14 proteins were assigned to this phylum. Our data suggest that m-xylene is assimilated by the Desulfobacteriaceae phylotype, whereas the role of the Epsilonproteobacterium in the consortium remained unclear.