We used a clone of the bacterium Escherichia coli previously adapted to 37 degrees C to found replicate populations propagated at constant 32 degrees C, constant 37 degrees C, constant 42 degrees C, and a daily alternation between 32 degrees and 42 degrees C. Several criteria indicate that 42 degrees was stressful for the ancestor, while 32 degrees and 37 degrees C were not. For example, 42 degrees C was within 1 degrees C of the limit for extinction, and yield was substantially reduced at this temperature. Adaptation was assayed by competing derived genotypes against their common ancestor at various temperatures. Bacteria adapted much more rapidly to 42 degrees C than to either lower temperature. Also, bacteria propagated in the alternating environment exhibited greater adaptation to 42 degrees C than to 32 degrees C. Adaptation was temperature-specific in all groups, but adaptation to 42 degrees C entailed little loss of fitness at lower temperatures. Nor did adaptation to 42 degrees C much extend the upper limit for population persistence, although we isolated more thermotolerant mutants by imposing hard selection. Thus, whereas the stressful 42 degrees C environment consistently led to more rapid adaptive evolution than did nonstressful regimes, superstressful temperatures caused either extremely rapid adaptive evolution or extinction. Although defining stress in general terms is difficult, one can evaluate specific criteria and test evolutionary hypotheses using appropriate experimental systems.