A comprehensive physiological and phylogenetic characterisation was carried out of "Thiobacillus ferrooxidans" m-1, an acidophilic iron-oxidizing bacterium first described over 25 years ago. Phylogenetically, strain m-1 is a gammaproteobacterium, most closely related to alkaliphilic Ectothiorhodospira spp. and only distantly to iron-oxidizing acidithiobacilli. Physiological examination confirmed that strain m-1 can grow autotrophically not only by ferrous iron oxidation but also, in contrast to previous reports, by oxidation of elemental sulfur, sulfide and tetrathionate, using either oxygen or ferric iron as terminal electron acceptor. The bacterium was also found to be thermo-tolerant, growing optimally at 38°C and up to a maximum of 47°C. Growth in liquid media required an external osmotic potential of >2 bar, and was optimal at ~5 bar, though no growth occurred where the medium osmotic potential was close to that of sea water (~26 bar). From this, it was concluded that strain m-1 is a moderate osmophile. Strain m-1 was also shown to be diazotrophic and tolerant of elevated concentrations of many metals typically found in mine-impacted environments. On the basis of these data, m-1 is proposed as the type strain of a new genus and species of bacteria, Acidiferrobacter thiooxydans (DSM 2392, JCM 17358).