P1 clones near the telomeres and centromeres of each mouse chromosome except Y have been selected from a mouse genomic library and mapped using fluorescencein situhybridization (FISH). Each clone was selected to contain a genetically mapped polymorphic DNA sequence as close as possible to the centromere or telomere of a chromosome. The genetic distance from the various P1 clones to the most distal genetically mapped polymorphic sequence ranged from 0 for about half of the clones to 6.7 cM for the probe at the telomere of chromosome 14. The average distance to the most distal or proximal chromosome marker was 1.5 cM. The use of FISH with these probes for mouse chromosome identification during comparative genomic hybridization is illustrated.