A new anaerobic, thermophilic, syntrophic, fatty-acid-oxidizing bacterium designated strain TGB-C1T was isolated from granular sludge in a thermophilic upflow anaerobic sludge blanket (UASB) reactor. The cells were slightly curved rods and were weakly motile. Spore formation was not observed. The optimal temperature for growth was around 55 degrees C and growth occurred in the range 45 to 60 degrees C. The pH range for growth was 5.8-7.5, and the optimum pH was 6.5-7.0. Crotonate was the only substrate that allowed the strain to grow in pure culture. However, in co-culture with the thermophilic, hydrogenotrophic Methanobacterium thermoautotrophicum strain delta H, the isolate could syntrophically oxidize saturated fatty acids with 4-10 carbon atoms, including isobutyrate. During the degradation of isobutyrate by the co-culture, isobutyrate was isomerized to butyrate, which was then oxidized. The strain was not able to utilize sulfate, sulfite, thiosulfate, nitrate, fumarate or Fe(III) as electron acceptor. The DNA base composition was 51.0 mol%. 16S rDNA sequence analysis revealed that the strain belongs to the family Syntrophomonadaceae, but it was only distantly related to other known species of beta-oxidizing syntrophs. Hence, the name Syntrophothermus lipocalidus is proposed for TGB-C1T as a new species of a new genus.