To determine the evolutionary origin of the human telomere sequence (TTAGGG)n, biotinylated oligodeoxynucleotides of this sequence were hybridized to metaphase spreads from 91 different species, including representative orders of bony fish, reptiles, amphibians, birds, and mammals. Under stringent hybridization conditions, fluorescent signals were detected at the telomeres of all chromosomes, in all 91 species. The conservation of the (TTAGGG)n sequence and its telomeric location, in species thought to share a common ancestor over 400 million years ago, strongly suggest that this sequence is the functional vertebrate telomere.