An isolate of an acidophilic archaeon, strain YT, was obtained from a bioleaching pilot plant. The organism oxidizes ferrous iron as the sole energy source and fixes inorganic carbon as the sole carbon source. The optimal pH for growth is 1.7, although growth is observed in the range pH 1.3 to 2.2. The cells are pleomorphic and without a cell wall. 16S rRNA gene sequence analysis showed this strain to cluster phylogenetically within the order 'Thermoplasmales' sensu Woese, although with only 89.9 and 87.2% sequence identity, respectively, to its closest relatives, Picrophilus oshimae and Thermoplasma acidophilum. Other principal differences from described species of the 'Thermoplasmales' are autotrophy (strain YT is obligately autotrophic), the absence of lipid components typical of the ' Thermoplasmales' (no detectable tetraethers) and a lower temperature range for growth (growth of strain YT occurs between 15 and 45 degrees C). None of the sugars, amino acids, organic acids or other organic compounds tested was utilized as a carbon source. On the basis of the information described above, the name Ferroplasma acidiphilum gen. nov., sp. nov. is proposed for strain YT within a new family, the Ferroplasmaceae fam. nov. Strain YT is the type and only strain of F. acidiphilum. This is the first report of an autotrophic, ferrous-iron-oxidizing, cell-wall-lacking archaeon.