THE WW domain is a new protein module with two highly conserved tryptophans that binds proline-rich peptide motifs in vitro. It is present in a number of signalling and regulatory proteins, often in several copies(1-3). Here we investigate the solution structure of the WW domain of human YAP65 (for Yes kinase-associated protein) in complex with proline-rich peptides containing the core motif PPxY (ref. 4). The structure of the domain with the bound peptide GTPPPPYTVG is a slightly curved, three-stranded, antiparallel beta-sheet. Two prolines pack against the first tryptophan, forming a hydrophobic buckle on the convex side of the sheet. The concave side has three exposed hydrophobic residues (tyrosine, tryptophan and leucine) which form the binding site for the ligand, A non-conserved isoleucine in the amino-terminal flanking region covers a hydrophobic patch and stabilizes the WW domain of human YAP65 irt vitro. The structure of the WW domain differs from that of the SH3 domain and reveals a new design for a protein module that uses stacked aromatic surface residues to arrange a binding site for proline-rich peptides.