An obligately anaerobic, hyperthermophilic, organoheterotrophic archaeon, strain 1633 T , was isolated from a terrestrial hot spring of the Uzon Caldera (Kamchatka Peninsula, Russia). Cells were regular cocci, 0.5-0.9 mm in diameter, with one flagellum. The temperature range for growth was 80-95 8C, with an optimum at 84 8C. Strain 1633 T grew on yeast extract, beef extract, peptone, cellulose and cellobiose. No growth was detected on other sugars or carbohydrates, organic acids, or under autotrophic conditions. The only detected growth products were CO 2 , acetate, and H 2 . The growth rate was stimulated by elemental sulfur, which was reduced to hydrogen sulfide. The in silico-calculated G+C content of the genomic DNA of strain 1633 T was 55.64 mol%. 16S rRNA gene sequence analysis placed strain 1633 T together with the non-validly published 'Thermogladius shockii' strain WB1 in a separate genus-level cluster within the family Desulfurococcaceae. Average nucleotide identity (ANI) results revealed 75.72 % identity between strain 1633 T and 'Thermogladius shockii' WB1. Based on these results we propose a novel genus and species with the name Thermogladius calderae gen. nov., sp. nov. The type strain of the type species is 1633 T (5DSM 22663 T 5VKM B-2946 T ).The vast majority of cultured Crenarchaeota have been isolated from terrestrial geothermal environments (Huber & Stetter, 2006). Located in Far East of Russia, Kamchatka Peninsula is a volcanically active area with hundreds of associated volcanoes and areas with geothermal activity, among which Uzon Caldera is the largest. Molecular studies have documented the presence of a significant number of cultured and uncultured Crenarchaeota in Uzon Caldera thermal pools. It was reported that Crenarchaeota inhabited not only high-temperature springs (Wemheuer et al., 2013;Chernyh et al., 2015), but also springs with moderate temperatures (Perevalova et al., 2008;Burgess et al., 2012;Menzel et al., 2015). At the time of writing, many thermophilic and hyperthermophilic crenarchaeotes have been isolated from hot springs of Uzon Caldera, including members of genera Desulfurococcus Slobodkina et al., 2015). Here we report the isolation of strain 1633 T obtained from a hot spring of the Uzon Caldera, and found to be closely related to 'Thermogladius shockii', a species with a name that was effectively, but never validly, published (Osburn & Amend, 2011).Strain 1633 T was isolated from a sample of mixed water and black mud from a freshwater hot spring (548 30.0479 N 1598 56.8519 E) located in the West Thermal Field of Uzon Caldera. The in situ water temperature and Abbreviations: ANI, average nucleotide identity; GDGT, glycerol dibiphytanyl glycerol tetraether.The GenBank/EMBL/DDBJ accession number for the complete genome sequence of Thermogladius calderae 1633 T is CP003531. The genome sequencing project of 'Thermogladius shockii' WB1 is available in the GenBank/EMBL/DDBJ databases under accession number PRJEB12686.