The naturally occurring aqueous Arsenic (As) and other toxic elements are found around the world. The present study concentrates on arsenic concentrations, speciation and related microbial diversity in a hydrothermal system in Western Turkey. The surface temperatures of hot springs reach up to 90 degrees C and deep well (reservoir) temperatures vary in the range of 40 to 230 degrees C. The elements such as As, B, Br, Ba, Cr, Fe, Mn, V and Zn are found in high concentration in hydrothermal waters. Hydrogeochemically, Seferihisar hot spring exhibited a Na-Cl water type. On the other hand, Karahayit, Pamukkale, Emirfaki, Alasehir and Sart exhibit a Ca-HCO3 water type and Citgol exhibited a Na-HCO3-SO4 water type. The arsenic (As) concentrations in geothermal waters of Western Anatolia have been detected to range from 0.03 mg/L to 1.5 mg/L, including Buharkent (Inalti) (1.50 +/- 0.005 mg/L), Kizildere (1.13 +/- 0.005 mg/L), Eynal (0.71 +/- 0.005 mg/L) and Saraykoy (0.06 +/- 0.004 mg/L). Arsenic (III) is the dominant species in geothermal water of Western Anatolia. The 16S rRNA gene sequences of bacterial diversity show that the thermophilic, sulfur/thiosulfate-oxidizing bacterium (Thiobacter subterraneus) is present in Kula geothermal water and mesophilic sulfur-and thiosulfate-oxidizing Sulfurovum lithotrophicum bacterium occurs in Saraykoy geothermal spring. Also, Bacillus fumarioli, (a thermophilic, aerobic endospore forming bacterium growing on (NH4)(2) SO4, MgSO4 and MnSO4 at 50-55 degrees C), Schlegelella thermodepolymerans and Methylocaldum szegediense are rich in geothermal water.