Two novel thermophilic and slightly acidophilic strains, Kam940(T) and Kam1507b, which shared 99 % 16S rRNA gene sequence identity, were isolated from terrestrial hot springs of the Uzon caldera on the Kamchatka peninsula. Cells of both strains were non-motile, regular cocci. Growth was observed between 55 and 85 degrees C, with an optimum at 65-70 degrees C (doubling time, 6.1 h), and at pH 4.5-7.5, with optimum growth at pH 5.5-6.0. The isolates were strictly anaerobic organotrophs and grew on a narrow spectrum of energy-rich substrates, such as beef extract, gelatin, peptone, pyruvate, sucrose and yeast extract, with yields above 10(7) cells ml(-1). Sulfate, sulfite, thiosulfate and nitrate added as potential electron acceptors did not stimulate growth when tested with peptone. H(2) at 100 % in the gas phase inhibited growth on peptone. Glycerol dibiphytanyl glycerol tetraethers (GDGTs) with zero to four cyclopentyl rings were present in the lipid fraction of isolate Kam940(T). The G+C content of the genomic DNA of strain Kam940(T) was 37 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the isolates were archaea of the phylum Crenarchaeota, only distantly related to the cultured members of the class Thermoprotei (no more than 89 % identity), and formed an independent lineage adjacent to the orders Desulfurococcales and Acidilobales and clustering only with uncultured clones from hot springs of Yellowstone National Park and Iceland as the closest relatives. On the basis of their phylogenetic position and novel phenotypic features, isolates Kam940(T) and Kam1507b are proposed to be assigned to a new genus and species, Fervidicoccus fontis gen. nov., sp. nov. The type strain of Fervidicoccus fontis is strain Kam940(T) (=DSM 19380(T) =VKM B-2539(T)). The phylogenetic data as well as phenotypic properties suggest that the novel crenarchaeotes form the basis of a new family, Fervidicoccaceae fam. nov., and order, Fervidicoccales ord. nov., within the class Thermoprotei.