Several anti-HIV aptamers adopt DNA quadruplex structures. Among these, "Hotoda's aptamer" (base sequence TGGGAG) was one of the first to be discovered. Although it has been the topic of some recent research, no detailed structural investigations have been reported. Here we report structural investigations on this aptamer and analogues with related sequences, by using UV, CD, and NMR spectroscopy as well as electrophoretic techniques. The addition of a 3'-end thymine has allowed us to obtain a single, investigable quadruplex structure. Data clearly point to the presence of an A-tetrad. Furthermore, the effects of the incorporation of an 8-methyl-2'-deoxyguanosine at the 5'-end of the G-run were investigated.