A slightly acidic hot spring named “Female Tower” (t=73.5°C, pH=6.64) is located in the Jifei Geothermal Field, Yunnan Province, southwestern China. The precipitates in the hot spring are composed of large amounts of calcite, aragonite and sulfur. Scanning electron microscopy (SEM) analyses reveal that the microbial mats were formed from various coccoid or rod-shaped filamentous microbes. Transmission electron microscopy (TEM) shows that the intracellular sulfur granules are commonly associated with these microbes. A culture-independent molecular phylogenetic analysis demonstrates that the majority of the bacteria in the spring are sulfur-oxidizing bacteria. In the spring water, H2S?concentration is up to 60 ppm, while SO42?concentration?is only about 10 ppm. We speculate that H2S might derive from sulfur-oxidizing bacteria in this hot spring water, leading to the intracellular formation of?sulfur granules.?Meanwhile, this reaction?increased?the pH in the micron-scale microdomains,?which?fosters the precipitation of calcium carbonate?in the microbial mats. The results of this study indicate that the sulfur-oxidizing bacteria might play an important role in?calcium carbonate?precipitation in slightly acidic?hot spring environments.