The mineral weathering capabilities ofThermothrix thioparawere investigated by scanning electron microscopy and energy dispersive X‐ray analysis.Thermothrix thioparais an extremely thermophilic, sulfur‐oxidizing bacterium that grows in a thermal spring whose principal minerals are calcium carbonate, pyrite, and sulfur. Crystals of these minerals were incubated in situ for periods up to eight days, removed, and examined. Results indicated thatT. thioparais partially responsible for weathering calcium carbonate by the production of sulfuric acid, thereby contributing to the formation of a porous tufa mound. Examination of ultravioletirradiated control crystals indicated that the sulfuric acid produced byT. thioparacaused solubilization of calcium carbonate even in the absence of direct bacterial colonization. Pyrite and sulfur were not visibly weathered, but instead were coated with calcium carbonate precipitate. During eight days incubation, growth forms ofT. thioparacolonizing the minerals progressed from unicells to filaments to nets of filaments. Bacteria other thanT. thioparaappeared after eight days, indicating an increased diversity.