Abstract: Moderately thermophilic sulfur-oxidizing bacteria resembling thiobacillus thiooxidans reffered to as MS (MS252,MS272,MS281,MS301) have been isolated from,acidic thermal habits in China. The optimum temperature for allculture appears to be 45℃; growth range is 25～55℃. The optimum pH is in the range of 3. 0～3. 5 ; growth rangeis 1. 0～5. 5 for strain MS252 and MS301, 1. 0～6. 0 for strain MS272 and MS281. The mol% G+C of the DNA is'57～60. The results indicate the strains MS have close relationship with Thiobacillus thiooxidans either obligateautotrophic or acidophilic. The latter has lower growth temperature (10～37℃ ) and lower mol % G+C of DNA(50～52). The new isolates Should be proposed as a new species of Thiobacillus thermothiooxidans sp. nov..