A novel thermoacidophilic iron-reducing Archaeon, strain NA-1, was isolated from a hot fumarole in Manza, Japan. Strain NA-1 could grow autotrophically using H-2 or S-0 as an electron donor and Fe3+ as an electron acceptor, and also could grow heterotrophically using some organic compounds. Fe3+ and O-2 served as electron acceptors for growth. However, S-0, NO3-, NO2-, SO4-2, Mn4+, fumarate, and Fe2O3 did not serve as electron acceptors. The ranges of growth temperature and pH were 60-90 degrees C (optimum: 80 degrees C) and pH 1.0-5.0 (optimum: pH 1.2-1.5), respectively. Cells were nearly regular cocci with an envelope comprised of the cytoplasmic membrane and a single outer S-layer. The crenarchaeal-specific quinone (cardariellaquinone) was detected, and the genomic DNA G + C content was 29.9 mol%. From 16S rDNA analysis, it was determined that strain NA-1 is closely related to Acidianus ambivalens (93.1%) and Acidianus infernus (93.0%). However, differences revealed by phylogenetic and phenotypic analyses clearly show that strain NA-1 represents a new species, Acidianus manzaensis, sp. nov., making it the first identified thermoacidophilic iron-reducing microorganism (strain NA-(T)= NBRC 100595 = ATCC BAA 1057).