A novel thermophilic, strictly anaerobic, heterotrophic bacterium, strain 2SM-2(T), was isolated from the Shengli oilfield, China. This organism was identified as a member of the order Thermotogales on the basis of its 16S rRNA gene sequence and the presence of an external membranous toga-like structure. Cells stained Gram-negative, were non-motile, appeared as irregular cocci 0.7-0.9 microm in diameter, and occurred in clusters of two to six cells, with cells located within a ballooning toga-like membrane. Its optimum temperature, pH and NaCl concentration for growth were 65 degrees C, 7.0 and 15 g l(-1), respectively. Under the optimum growth conditions, the doubling time was approximately 105 min. Strain 2SM-2(T) fermented a variety of simple and complex substrates such as glucose, acetate, methanol, starch and peptone while reducing elemental sulfur, sulfate and thiosulfate. The end products identified during growth on glucose were acetate, lactate, L-alanine, H2 and CO2. The DNA G+C content of this organism was 36.4 mol%. The results of 16S rRNA gene-based sequence comparisons revealed that the strain represented a new lineage within the family Thermotogaceae of the order Thermotogales. Based on the phenotypic and phylogenetic characteristics, it is proposed that this organism represents a novel species in a new genus within the family Thermotogaceae, for which the name Thermococcoides shengliensis gen. nov., sp. nov. is proposed. The type strain is 2SM-2(T) (=ACCC 00496(T)=DSM 22460(T)).