To test whether expression of the inducible heat-shock protein Hsp70 increases under selection for inducible thermotolerance in Drosophila melanogaster, we performed artiﬁcial selection on replicate sets of Drosophila lines founded from two independent populations. Selection entailed pretreatment at 36C to induce thermotolerance and Hsp70 expression, followed by a more severe heat shock, whose temperature varied between sexes and among generations to achieve 50% mortality. Inducible thermotolerance increased slowly and continuously in selected lines and was 37%-50% greater than in controls after 10-11 generations. Lines founded from the two populations differed in their coevolution of Hsp70 expression. In lines founded from Evolution Canyon, Israel, Hsp70 level initially increased and thereafter was unchanged; replicate lines exhibited two temporal patterns of response to selection. In lines founded from Australia, Hsp70 levels increased throughout selection. In both cases, however, the increase in Hsp70 level averaged only 15%, suggesting that pleiotropy in Hsp70 function constrains evolutionary increase in its expression.