Ferroglobus placidus was discovered to oxidize completely the aromatic amino acids tyrosine, phenylalanine and tryptophan when Fe(III) oxide was provided as an electron acceptor. This property had not been reported previously for a hyperthermophilic archaeon. It appeared that F. placidus follows a pathway for phenylalanine and tryptophan degradation similar to that of mesophilic nitrate-reducing bacteria, Thauera aromatica and Aromatoleum aromaticum EbN1. Phenylacetate, 4-hydroxyphenylacetate and indole-3-acetate were formed during anaerobic degradation of phenylalanine, tyrosine and tryptophan, respectively. Candidate genes for enzymes involved in the anaerobic oxidation of phenylalanine to phenylacetate (phenylalanine transaminase, phenylpyruvate decarboxylase and phenylacetaldehyde : ferredoxin oxidoreductase) were identified in the F. placidus genome. In addition, transcription of candidate genes for the anaerobic phenylacetate degradation, benzoyl-CoA degradation and glutaryl-CoA degradation pathways was significantly upregulated in microarray and quantitative real-time-PCR studies comparing phenylacetate-grown cells with acetate-grown cells. These results suggested that the general strategies for anaerobic degradation of aromatic amino acids are highly conserved amongst bacteria and archaea living in both mesophilic and hyperthermophilic environments. They also provided insights into the diverse metabolism of Archaeoglobaceae species living in hyperthermophilic environments.