The structure of a highly conserved complex between a 58-nucleotide domain of Large subunit ribosomal RNA and the RNA-binding domain of ribosomal protein L11 has been solved at 2.8 angstrom resolution. It reveals a precisely folded RNA structure that is stabilized by extensive tertiary contacts and contains an unusually Large core of stacked bases. A bulge Loop base from one hairpin of the RNA is intercalated into the distorted major groove of another helix; the protein Locks this tertiary interaction into place by binding to the intercalated base from the minor groove side. This direct interaction with a key ribosomal RNA tertiary interaction suggests that part of the role of L11 is to stabilize an unusual RNA fold within the ribosome.