Site-specific proteases and antisera to the amino terminus of villin have been used to show that villin is organized into seven protease-resistant domains. Six are contained in the amino-terminal Mr 87,000 villin core, a Ca2+-regulated actin-severing fragment, whereas the carboxyl-terminal domain includes the villin "headpiece," a fragment involved in bundling of actin filaments. Ca2+ inhibits proteolytic cleavage between domains in the amino-terminal half of villin. The protein sequence of villin deduced from a single cDNA clone contains a conserved sequence that is repeated six times and is found in each domain of the villin core. The conserved repeats are found in other actin-severing proteins but not in the villin headpiece. Our results suggest that actin-severing proteins are organized around a common Mr 14,000-17,000 domain.