A 42-cM map of proximal mouse chromosome 9, including eight loci defined by restriction fragment length variants, has been generated. Linkage was established by haplotype analyses of 114 interspecific backcross mice and indicated the following gene order: (centromere) Pvs-5.3 cM-Icam-1/Ldlr-18.4 cM-Thy-1-1.8 cM-Ncam-0.9 cM-Hexa-7.9 cM-Gsta-7.9 cM-Trf. Three of these loci, Pvs, Icam-1, and Hexa, have not been mapped previously. Together with previous mapping studies the current results suggested that chromosomal segments of mouse chromosomes 7 and 9 and chromosomal segments of human chromosomes 11, 15, and 19 derive from a single putative primordial chromosome. The studies support the postulate that detailed analysis of chromosome organization will be useful in defining events in mammalian evolution.