"pyrobaculum yellowstonensis" strain wp30 was obtained from an elemental sulfur sediment (joseph's coat hot spring [jchs], 80°c, ph 6.1, 135 μm as) in yellowstone national park (ynp), usa and is a chemoorganoheterotroph and requires elemental sulfur and/or arsenate as an electron acceptor. growth in the presence of elemental sulfur and arsenate resulted in the formation of thioarsenates and polysulfides. the complete genome of this organism was sequenced (1.99 mb, 58% g+c content), revealing numerous metabolic pathways for the degradation of carbohydrates, amino acids, and lipids. multiple dimethyl sulfoxide-molybdopterin (dmso-mpt) oxidoreductase genes, which are implicated in the reduction of sulfur and arsenic, were identified. pathways for the "de novo" synthesis of nearly all required cofactors and metabolites were identified. the comparative genomics of "p. yellowstonensis" and the assembled metagenome sequence from jchs showed that this organism is highly related (∼95% average nucleotide sequence identity) to "in situ" populations. the physiological attributes and metabolic capabilities of "p. yellowstonensis" provide an important foundation for developing an understanding of the distribution and function of these populations in ynp.