Optimal temperatures for elemental sulfur oxidation were determined for Sul/olobus acidocaldarius populations in a variety of hot, acid springs in Yellow- stone National Park, Wyoming. Cultures were obtained from some of the springs by enrichment and isolation at different incubation temperatures, and the optimal temperatures for these were also determined. The optimum temperature for sulfur oxidation was the same as that for 14C0~ incorporation and for the rate of growth of several isolates. For populations in springs of 57--80~ the optimal temperatures were either equal to or higher than the habitat temperature. However, the optima for bacteria in the high-temperature springs (90--92~ were considerably lower than the observed habitat temperature, suggesting an inability to adapt to tempera- tures near boiling. These results, as well as the properties of the bacteria isolated from the sites, suggest that a limited number of temperature strains of Sul/olobus exist in nature. A mixture of strains occurred at severaI of the sites, pointing to an unexpected heterogeneity in these well-mixed springs.