Unusual DNA structures involving four guanines in a planar formation (guanine tetrads) are formed by guanine-rich (G-rich) telomere DNA and other G-rich sequences (reviewed in (1)) and may be important in the structure and function of telomeres. These structures result from intrastrand and/or interstrand Hoogsteen base pairs between the guanines. We used the telomeric repeat of Chlamydomonas reinhardtii, TTTTAGGG, which contains 3 guanines and has a long interguanine A + T tract, to determine whether these sequences can form intrastrand and interstrand guanine tetrads. We have found that ss (TTTTAGGG)4 can form intrastrand guanine tetrads that are less stable than those formed by more G-rich telomere sequences. They are not only more stable, but also more compact, they are more stable in the presence of K+ than they are in the presence of Na+. While ds oligonucleotides with ss 3' overhangs of (TTTTAGGG)2 can be observed to associate as dimers, formation of this interstrand guanine tetrad structure occurs to a very limited extent and requires very high G-strand concentration, high ionic strength, and at least 49 hours of incubation. Our results suggest that, if telomere dimerization occurs in vivo, it would require factors in addition to the TTTTAGGG telomere sequence.