TELOMERE structure and behaviour is less well understood in vertebrates than it is in ciliates and yeasts (reviewed in ref. 1). Like all other eukaryotic chromosomes, those of vertebrates terminate in an array of a short repeated sequence. In vertebrates this sequence is (TTAGGG)_n, as shown by in situ hybridization^2,3. In humans, these terminal repeats are heterogeneous in length, averaging about 10 kilobases in blood cells^4–6. Here we report the structure and inheritance of the terminal repeats present at mouse telomeres. The (TTAGGG)_n tracts are many times larger than those present at human telomeres. Because of their constancy in length through somatic cell divisions, they are resolved as multiple discrete restriction fragments of up to 150 kilobases. Strikingly, this banding pattern is highly polymorphic within populations of inbred mice, suggesting an unusually high mutation rate. Indeed, although the banding pattern is inherited in a largely mendelian fashion, (TTAGGG)_n tracts of new size appear frequently in family studies.