Shank proteins are a new family of scaffold proteins interacting with various membrane and cytoplasmic proteins. Shank contains multiple protein-protein interaction sites, including ankyrin repeats, an SH3 domain, a PDZ domain, a long proline-rich region and an SAM domain. The PDZ domain of Shank binds to the C-terminus of guanylate kinase-associated protein (GKAP). The PDZ domain of Shank1 from Rattus norvegicus and its complex with the C-terminal octapeptide of GKAP were crystallized at 294 K using polyethylene glycol 20 000 and 6000 as precipitants. Diffraction data sets from a peptide-free crystal and a complex crystal were collected to 1.8 and 3.2 Angstrom resolution, respectively, using synchrotron radiation. The peptide-free crystal belongs to space group P2(1), with unit-cell parameters a = 42.0, b = 50.3, c = 51.8 Angstrom, beta = 106.3degrees. The complex crystal belongs to space group P21 21 21, with unit-cell parameters a = 89.4, b = 97.5, c = 108.3 Angstrom.