PDZ domains (also known as DHR domains or GLGF repeats) are similar to 90-residue repeats found in a number of proteins implicated in ion-channel and receptor clustering, and the linking of receptors to effector enzymes(1). PDZ domains are protein-recognition modules; some recognize proteins containing the consensus carboxy-terminal tripeptide motif S/TXV with high specificity(2-4). Other PDZ domains form homotypic dimers : the PDZ domain of the neuronal enzyme nitric oxide synthase binds to the PDZ domain of PSD-95, an interaction that has been implicated in its synaptic association(5). Here we report the crystal structure of the third PDZ domain of the human homologue of the Drosophila discs-large tumour-suppressor gene product, DlgA. It consists of a five-stranded antiparallel beta-barrel flanked by three alpha-helices. A groove runs over the surface of the domain, ending in a conserved hydrophobic pocket and a buried arginine; we suggest that this is the binding site for the C-terminal peptide.