Phenol degradation under methanogenic conditions has long been studied, but the anaerobes responsible for the degradation reaction are still largely unknown. An anaerobe, designated strain UIT, was isolated in a pure syntrophic culture. This isolate is the first tangible, obligately anaerobic, syntrophic substrate-degrading organism capable of oxidizing phenol in association with an H-2-scavenging methanogen partner. Besides phenol, it could metabolize p-cresol, 4-hydroxybenzoate, isophthalate, and benzoate. During the degradation of phenol, a small amount of 4-hydroxybenzoate (a maximum of 4 mu M) and benzoate (a maximum of 11 mu M) were formed as transient intermediates. When 4-hydroxybenzoate was used as the substrate, phenol (maximum, 20 mu M) and benzoate (maximum, 92 mu M) were detected as intermediates, which were then further degraded to acetate and methane by the coculture. No substrates were found to support the fermentative growth of strain UIT in pure culture, although 88 different substrates were tested for growth. 16S rRNA gene sequence analysis indicated that strain UIT belongs to an uncultured clone cluster (group TA) at the family (or order) level in the class Deltaproteobacteria. Syntrophorhabdus aromaticivorans gen. nov., sp. nov., is proposed for strain UIT, and the novel family Syntrophorhabdaceae fam. nov. is described. Peripheral 16S rRNA gene sequences in the databases indicated that the proposed new family Syntrophorhabdaceae is largely represented by abundant bacteria within anaerobic ecosystems mainly decomposing aromatic compounds.