The structure of ribosomal protein S7 from Bacillus stearothermophilus has been solved at 2.5 Å resolution by multiwavelength anomalous diffraction method using selenomethionyl-substituted proteins. The molecule consists of a helical hydrophobic core domain and a β-ribbon arm extending from the hydrophobic core. The helical core domain is composed of a pair of entangled helix-turn-helix motifs; the fold of the core is similar to that of a DNA architectural factor. Highly conserved basic and aromatic residues are clustered on one face of the S7 molecule and create a 16S rRNA contact surface. The molecular structure of S7, together with the results of previous cross-linking experiments, suggests how this ribosomal protein binds to the 3' major domain of 16S rRNA and mediates the folding of 16S rRNA to create the ribosome decoding center.