I searched for quantitative trait loci (QTL) for upper thermal tolerance using linkage mapping approach in rainbow trout, ('Oncorhynchus mykiss'). An existing rainbow trout experimental family set was expanded by genotyping 32 new microsatellite markers and one candidate gene marker in three paternal half-sib families. Information on the chromosomal location of new markers in the genome of rainbow trout and additional information on meioses of previously mapped markers resulted in the detection of a new and significant QTL on linkage group RT-9, in the interval between marker OMM1096 and the candidate gene Hsp70aUoG. This was based on the segregation of both maternal and paternal alleles in lot 22, where marker Hsp700708a was derived from the untranslated region of HSP70 (heat shock protein70) gene sequence. Another significant QTL was detected on RT-31 with marker OMM1080 in family 25 using paternal allelic information whereas five suggestive QTLs were detected on RT-22, RT-15, RT-27, RT-2 and RT-26. Results of my research suggest that QTL's with large phenotypic effect may account for the majority of the heritable phenotypic expression of a trait.