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The way populations respond to selection can be altered when populations are acclimated prior to selection. To examine this possibility, the responses of replicate lines of Drosophila melanogaster and D. simulans to selection for increased resistance to cold were compared. Flies were selected without hardening or after they had been hardened by holding them at 4°C for one hour. The selection response in both species was much greater when flies were not cold‐hardened. Cold resistance in both sets of selected lines reached a plateau after a few generations. Surprisingly, continued selection for increased resistance resulted in decreasing levels of resistance. This decrease was no longer evident after selection had been relaxed for a generation, suggesting cross‐generation effects. The magnitude of the cross‐generation effects increased with additional generations of selection. Cross‐generation effects …