A new aerobic, obligately chemolithoautotrophic, thermophilic, sulfur-oxidizing bacterium, Thermothrix azorensis, was isolated from a hot spring on Sao Miguel Island in the Azores. The cells of this organism are gram negative, nonsporulating, and rod shaped. Filament formation appears to occur as a response to nonoptimal growth conditions. Growth occurs at 63 to 86 degrees C, and the optimum temperature is 76 to 78 degrees C. The optimum pH range for growth is 7.0 to 7.5. The G+C content of the DNA of our isolate is 39.7 mol%. This isolate uses thiosulfate, tetrathionate, hydrogen sulfide, and elemental sulfur as energy sources. Of particular interest are the absence of Calvin cycle enzymes and the initial appearance of sulfide during the lag phase of growth of aerobic cultures grown on elemental sulfur. The subsequent formation of thiosulfate is followed by oxidation of the thiosulfate to sulfate. T. azorensis differs from the only other Thermothrix species that has been described, Thermothrix thiopara, by having higher optimum and maximum growth temperatures, by being an obligate chemolithoautotroph, and by its close but separate position on a 16S rRNA sequence-based phylogenetic tree. Our T. azorensis isolate has been deposited in the American Type Culture Collection as strain ATCC 51754T (T = type strain).