Regular exposure to a given stressor poses selective pressure towards increased resistance. In high temperature environments this can be mediated through physiological and biochemical adjustments that lead to increased resistance to heat coma. Such adjustments may influence complex traits such as locomotor activity through cascading effects. Alternatively, changes in locomotor activity may change the exposure to the stressor and, thereby, the importance of the physiological and biochemical adjustments. In this experiment we analysed how selection for heat shock resistance may affect locomotor activity at high temperatures in Drosophila melanogaster. We compared 2 lines selected for heat shock (HS) and cold shock (CS) resistance at 4 constant high temperatures (28 to 38 degrees C). We also tested the importance of heat shock protein (Hsp) expression by comparing locomotor activity at the same temperatures of a heat-sensitive mutant line (Hsf(0)), where the heat shock factor (Hsf) is non-functional at high temperatures, with that of a rescued mutant line (Hsf(+)) with a functional Hsf re-inserted. At moderately stressful temperatures (28 and 34 degrees C), flies from the Hsf(0) line were more active than flies from the Hsf(+) line, whereas this was reversed at higher temperatures. In the selection lines, the CS flies were more active at 28 degrees C but this was reversed at 34 and 36 degrees C, whereas no difference was observed at the most stressful temperature (38 degrees C). This suggests that the lines selected for increased cold tolerance were more stressed at moderately high temperatures and, therefore, more active in the attempt to escape the stress, whereas at higher temperatures, their locomotor activity was more compromised than in flies from the lines selected for increased heat resistance.