A general experimental design that allows mapping of a quantitative trait locus (QTL) into a 1-cM interval is presented. The design consists of a series of strains, termed "interval-specific congenic strains (ISCS)". Each ISCS is recombinant at a specific 1-cM sub-interval out of an ordered set of sub-intervals, which together comprise a wider interval, to which a QTL was previously mapped. It is shown that a specific and previously detected QTL of moderate or even small effect can be accurately mapped into a 1-cM interval in a program involving a total of no more than 1000 individuals. Consequently, ISCS can serve as the ultimate genetic mapping procedure before the application of physical mapping tools for positional cloning of a QTL.