We have solved at 1.07 � resolution the X-ray crystal structure of a polyriboadenylic acid (poly(rA)) parallel and continuous double helix. Fifty-nine years ago, double helices of poly(rA) were first proposed to form at acidic pH. Here, we show that 7-mer oligo(rA), i.e. rA<
sub>
7<
/sub>
, hybridizes and overlaps in all registers at pH 3.5 to form stacked double helices that span the crystal. Under these conditions, rA<
sub>
7<
/sub>
forms well-ordered crystals, whereas rA<
sub>
6<
/sub>
forms fragile crystalline-like structures, and rA<
sub>
5<
/sub>
, rA<
sub>
8<
/sub>
and rA<
sub>
11<
/sub>
fail to crystallize. Our findings support studies from ~50 years ago: one showed using spectroscopic methods that duplex formation at pH 4.5 largely starts with rA<
sub>
7<
/sub>
and begins to plateau with rA<
sub>
8<
/sub>
another proposed a so-called ?staggered zipper? model in which oligo(rA) strands overlap in multiple registers to extend the helical duplex. While never shown, protonation of adenines at position N1 has been hypothesized to be critical for helix formation. Bond angles in our structure suggest that N1 is protonated on the adenines of every other rAMP?rAMP helix base pair. Lastly, our data offer new insights into poly(rA) duplex formation that may be useful in developing a pH sensor.