We have used 54 microsatellite markers to search for quantitative trait loci (QTL) associated with spawning time in rainbow trout (Oncorhynchus mykiss). Linkage tests have been conducted in a backcross family produced by crossing spring and fall spawning stains in Ontario, Canada. We have mapped thirteen QTL markers for spawning time representing seven Linkage groups. Eight of these QTL markers from five linkage groups show repeatable effects in two sampling years. These results suggest this trait is highly polygenic in rainbow trout. (C) 1999 Elsevier Science B.V. All rights reserved.