In the past workers have isolated several pseudomonad strains which have been used for studies of anaerobic aromatic metabolism. The best studied of these strains are strains KB 740T (T = type strain) and K 172T. The taxonomic positions of these two organisms were determined by classical methods, including experiments to determine substrate spectrum, quinone type, and total fatty acid composition. Our results clearly excluded these strains from the authentic genus Pseudomonas, which belongs to the gamma subclass of the Proteobacteria. Instead, the properties of these organisms indicated that they belong to the beta subclass of the Proteobacteria. The sequences of the 16S ribosomal DNA genes confirmed this conclusion and indicated that strain K 172T represents a new species of the genus Thauera, Thauera aromatica, and that strain KB 740T represents a new species of the genus Azoarcus, Azoarcus evansii.