Strictly anaerobic, thermophilic bacteria (strains SL24(T), SL25(T), SL27, SL29 and SL32) were isolated from a deep, continental oil reservoir in Western Siberia (Russia). These motile, rod-shaped organisms were surrounded by a sheath-like structure, a feature characteristic of the Thermotogales. On the basis of partial 16S rDNA sequences (500 nucleotides), strains SL25(T), SL27, SL29 and SL32 were identical. Therefore, only strains SL24(T) and SL25(T) were studied in detail. The optimum temperature for growth of both strains was 55degreesC. Their optimum pH for growth was 7.5 and their optimum NaCl concentration was between 20 and 30 g l(-1). The novel isolates reduced elemental sulfur and cystine, but not thiosulfate or sulfate, to hydrogen sulfide. The G+C contents of the genomic DNA of strains SL24(T) and SL25(T) were respectively 35 and 33 mol%. Phylogenetically, both strains are most closely related to Petrotoga miotherma, there being 98.9-99.4% similarity between their 16S rDNA sequences. Phenotypic properties and DNA-DNA hybridization experiments indicate that the strains belong to two novel species, for which the names Petrotoga olearia (type strain SL24(T)=DSM 13574(T)=JCM 11234(T)) and Petrotoga sibirica (type strain SL25(T)=DSM 13575(T)=JCM 11235(T)) are proposed.