Two loci FRI (FRIGIDA) and KRY (KRYOPHILA) have previously been identified as having major influences on the flowering time of the late-flowering, vernalization-responsive Arabidopsis ecotype, Stockholm. We report here on the mapping and subsequent analysis of these two loci. FRI was mapped to the top of chromosome 4 between markers w122 and m506, using restriction fragment length polymorphism (RFLP) analysis. Due to lack of segregation in of the late-flowering phenotype under the environmental conditions used, KRY could only be localized, by "subtractive genotyping", to chromosome 5 or part of chromosome 3. The map position of FRI indicates that it is not allelic to any of the late-flowering loci identified by mutagenesis of the early-flowering ecotype Landsberg erecta. The late-flowering phenotype conferred by the Stockholm allele of FRI is modified (towards earlier flowering) by Landsberg erecta alleles at an unknown number of loci, perhaps accounting for the absence of fri mutations among mutant lines recovered in Landsberg erecta.