Many biologically and economically important traits in plants and animals are quantitative/multifactorial, being controlled by several quantitative trait loci (QTL). QTL are dif®cult to locate accurately by conventional methods using molecular markers in segregating populations, particularly for traits of low heritability or for QTL with small effects. In order to resolve this, large (often unrealistically large) populations are required. In this paper we present an alternative approach using a specially developed resource of lines that facilitate QTL location ®rst to a particular chromosome, then to successively smaller regions within a chromosome (`0.5 cM) by means of simple comparisons among a few lines. This resource consists of`Stepped Aligned Inbred Recombinant Strains' (STAIRS) plus single whole Chromosome Substitution Strains (CSSs). We explain the analytical power of STAIRS and illustrate their construction and use with Arabidopsis thaliana, although the principles could be applied to many organisms. We were able to locate¯owering QTL at the top of chromosome 3 known to contain several potential candidate genes.