Seven strains of extremely thermophilic, obligately anaerobic cocci were isolated from the terrestrial hot springs of Kamchatka and Kunashir [1]. The temperature optimum for growth of the new isolates was 90‐ 92 ° C. Investigation of the new isolates revealed their similarity in all major diagnostic features. The beststudied representative of this group, strain Z-533, was described as the type strain of a new species of sulfurreducing archaea, Desulfurococcus amylolyticus [2]. In order to determine the phylogenetic position of this organism and confirm its affiliation to the genus Desulfurococcus , the nucleotide sequence of its 16S rRNA gene selectively amplified in vitro with the use of universal primers was analyzed [3]. The amplification was carried out in a reaction buffer composed of 1.5 mM MgCl 2 , 50 mM KCl, 10 mM Tris‐HCl (pH 8.3), and 0.001% gelatin. The reaction mixture (100 μ l) contained standard concentrations of dNTPs and equimolar concentrations of pA and pH' primers. Thirty amplification cycles with the following temperature profile