The telomeres of most eukaryotes contain tandemly-repeated DNA sequences, with a cluster of G residues on one strand. Recent studies showed that the Oxytricha telomeric DNA oligonucleotide, d(G 4T 4G 4), dimerizes to form a quadruplex in the presence of Na + or K +. We have observed that the oligonucleotide d(G 4T 4G 4) does not dimerize in the presence of Li + ion at low sample concentrations. In the monomeric state, this molecule forms a simple foldback hairpin structure containing G · G reverse Hoogsteen basepairs in the stem region. This hairpin structure has a thermal stability which is well reconciled with telomere functions in vivo.