Headpiece (HP) is a 76-residue F-actin-binding module at the C terminus of many cytoskeletal proteins. Its 35-residue C-terminal subdomain is one of the smallest known motifs capable of autonomously adopting a stable, folded structure in the absence of any disulfide bridges, metal ligands, or unnatural amino acids. We report the three-dimensional solution structures of the C-terminal headpiece subdomains of human villin (HVcHP) and human advillin (HAcHP), determined by two-dimensional 1H-NMR. They represent the second and third structures of such C-terminal headpiece subdomains to be elucidated so far. A comparison with the structure of the chicken villin C-terminal subdomain reveals a high structural conservation. Both C-terminal subdomains bind specifically to F-actin. Mutagenesis is used to demonstrate the involvement of Trp 64 in the F-actin-binding surface. The latter residue is part of a conserved structural feature, in which the surface-exposed indole ring is stacked on the proline and lysine side chain embedded in a PXWK sequence motif. On the basis of the structural and mutational data concerning Trp 64 reported here, the results of a cysteine-scanning mutagenesis study of full headpiece, and a phage display mutational study of the 69-74 fragment, we propose a modification of the model, elaborated by Vardar and coworkers, for the binding of headpiece to F-actin.