Chemolithotrophic metabolism fuels primary production in many hydrothermal ecosystems, representing energy conservation strategies that may be independent of sunlight. Many autotrophic metabolic pathways in geothermal communities depend on H₂; however, in this paper we focus on anaerobic carboxydotrophs that are capable of using CO as a carbon and energy source, producing H₂ and CO₂. We have recently completed the genome sequence of an autotrophic carboxydotroph, Carboxydothermus hydrogenoformans, which appears to be a specialist in CO-oxidation, encoding five unlinked genetic loci capable of expressing CO dehydrogenase. Because of their global occurrence in geothermal environments, we propose that CO-utilizing organisms play important roles in anaerobic microbial consortia. To understand the role of CO-utilizing bacteria in Yellowstone hot springs we isolated and described a novel H₂-producing bacterium (strain Nor1) from an Fe-rich site in Norris Geyser Basin, YNP. Strain Nor1 is a low-G+C Gram-positive bacterium belonging to the division Firmicutes, and grows chemolithotrophically at 75°C on CO (generation time 1.5 h), producing equimolar quantities of H₂ and CO₂. Strain Nor1 is also capable of chemoautotrophic growth on Fe^III and Se, as well as growing heterotrophically on glucose and several sugars, producing acetate, H₂, and CO₂. We have proposed that strain Nor1 be assigned to a new genus, Thermosinus gen. nov. The type species is Thermosinus carboxydivorans sp. nov. (type strain, Nor1T =DSM 14886T; Sokolova et al. 2004). It is evident that CO-dependent hydrogenogenic metabolism occurs in a diverse phylogenetic context and can be accompanied by a varied repertoire of alternative trophic strategies.