The taxonomic status of Sulfolobus hakonensis Takayanagi et al. 1996 was re-evaluated by fresh determinations of the 16S rDNA sequence and G+C content of the genomic DNA of the type strain, HO1-1(T). The 16S rDNA sequence of strain HO1-1(T) showed 98 % similarity to those of two Metallosphaera species and only </=92 % similarity to those of other Sulfolobus species. The DNA G+C content (46.2 mol%) is in accordance with those of Metallosphaera species. In addition, strain HO1-1(T) shares some phenotypic properties with Metallosphaera species; however, it can be differentiated from them by its capacity to utilize FeS and tetrathionate and the absence of flagella. Therefore, it is proposed that Sulfolobus hakonensis should be transferred to the genus Metallosphaera as Metallosphaera hakonensis comb. nov.