Two repetitive DNA fragments located on the mouse X chromosome are described. The fragments were isolated from a lambda phage library enriched in X-chromosomal sequences by flow sorting. Both fragments, which are repeated 20 to 50 times in the genome, were mapped to the mouse X chromosome by Southern blot hybridization to DNA from hybrid cells retaining the mouse X chromosome, by dosage analysis, and by in situ hybridization to mouse chromosomes. In mouse strain C57BL/10BK, one fragment appeared to be located only on the X chromosome, while the other fragment had homologous sequences on chromosome 11 in addition to the X chromosome. The latter fragment showed DNA variants between mouse strains, which are potentially useful for mapping. Both fragments cross-hybridized to another mouse species: Mus caroli. In this species, each fragment appeared to be located on the X chromosome, indicating that some X-chromosome repetitive sequences are partially conserved. In addition, one fragment cross-hybridized to human DNA.