A three-dimensional tracking microscope was used to quantify the effects of temperature (50 to 80°C) and pH (2 to 4) on the motility of Sulfolobus acidocaldarius, a thermoacidophilic archaeon. Swimming speed and run time increased with temperature but remained relatively unchanged with increasing pH. These results were consistent with reported changes in the rate of respiration of S. acidocaldarius as a function of temperature and pH. Cells exhibited a forward-biased turn angle distribution with a mean of 54°. Cell trajectories during a run were in the shape of right-handed helices. A cellular dynamics simulation was used to test the hypothesis that a population of S. acidocaldarius cells could distribute preferentially in a spatial temperature gradient due to variation in swimming speed. Simulation results showed that a population of cells could migrate from a higher to a lower temperature in the presence of sharp temperature gradients. This simulation result was achieved without incorporating the ability of cells to sense a temporal thermal gradient; thus, the response was not thermotactic. We postulate that this temperature-sensitive motility is one survival mechanism of S. acidocaldarius that allows this organism to move away from lethal hot spots in its hydrothermal environment.