Psychrotrophic and mesophilic isolates of Thiobacillus ferrooxidans were examined for their ability to survive at temperatures above the T-max, below the T-min, and at -15 degrees C after a slow freeze. There were no thermoduric strains among those studied; the viable counts decreased by two to five orders of magnitude in 24 h, following exposure to a supermaximum temperature (2-4 degrees C above the T-max). Strain F1, when exposed to progressively higher temperatures, predictably showed increasingly rapid rates of death. When strain S2 was exposed to 2 degrees C, a temperature below its T-min but still above freezing, there was little change in the viable counts over the 38-day observation period. When the various strains were subjected to a slow freeze at -15 degrees C, the cells died quite rapidly with the percentage survival among the strains varying from .0006% to .0155% after 24 h. A survival curve for strain Al showed that the number of viable cells decreased by approximately three orders of magnitude in the first 4-6 h, and a further three orders of magnitude over the next 40 h.