Knockdown Resistance to High Temperature (KRHT) is an adaptive trait of thermotolerance in insects. An interval mapping was performed on chromosome X of Drosophila melanogaster to search for quantitative trait loci (QTL) affecting KRHT. A backcross population was obtained from two lines that dramatically differ for KRHT. Microsatellites were used as markers. Composite interval mapping identified a large-effect QTL in the region of band 10 where putative candidate genes map. To further test for this QTL a set of recombinant (but non-inbred) lines was obtained from backcrosses between the parental lines used for the interval mapping. Recombinant line analysis confirmed that one QTL is targeted by band 10. We identify and discuss candidate loci contained within our QTL region.