The novel isolate reduced elemental sulfur ana cystine. but not thiosulfate sulfate, to hydrogen sulfide, The G+C content of the genomic DNA was 30.0 mol%, As determined by 16S rDNA sequence analysis, this organism Three strictly anaerobic, thermophilic bacteria (SL31(T), SL30 and MLM39636) were isolated from a deep continental oil reservoir in Western Siberia (Russia), Following the mid-exponential phase of growth, the non-motile rod-shaped organisms were surrounded by a sheath-like structure, As DNA-DNA hybridizations showed that these strains were highly related genomically, only strain SL31T was studied in detail, The temperature range for growth of strain SL31T was between 45 and 75 degreesC, with optimum growth at 70 degreesC, its optimum ph and NaCl concentration for growth were pH 7.5 and 20-30 g l(-1), respectively. The novel isolate reduced elemental sulfur and cystine, but not thiosulfate or belonged to the genus Thermosipho. DNA-DNA hybridization levels between strain SL31T and type strains of the previously described species of Thermosipho were less than 10%, On the basis of physiological and molecular properties, it is proposed that this organism should be placed in a new species, Thermosipho geolei sp, nov, The novel organism represents the first species of the genus Thermosipho that has been isolated from a petroleum reservoir, The type strain is SL31(T) (= DSM 13256(T) = JCM 10986(T)).