Small protein B (SmpB) is required for trans-translation, binding specifically to tmRNA. We show here the solution structure of SmpB from an extremely thermophilic bacterium, Thermus thermophilus HB8, determined by heteronuclear nuclear magnetic resonance methods. The core of the protein consists of an antiparallel beta-barrel twisted up from eight beta-strands, each end of which is capped with the second or third helix, and the first helix is located beside the barrel. Its C-terminal sequence (20 residues), which is rich in basic residues, shows a poorly structured form, as often seen in isolated ribosomal proteins. The results are discussed in relation to the oligonucleotide binding fold.