A 95,000 dalton actin-binding polypeptide, villin, has been purified to 98% homogeneity from brush border cytoskeletons of chicken intestinal epithelial cells. In vitro, this protein exerts control over the polymerization of actin. In the presence of villin, the lag phase preceding detectable actin polymerization is shortened and the steady state equilibrium viscosity is reduced in proportion to the amount of villin present. A molar ratio of villin:actin of 1:40 results in a 70% reduction of the Ostwald viscosity. Significant effects can be detected at a ratio of 1:600. These ratios are physiologically relevant because the ratio of villin:actin in brush borders is 1:13 and in isolated microvilli is 1:9-12. Reduction of viscosity is mirrored by an increase in the amount of protein which fails to sediment at 150,000 X g for 60 min. An assay of the nonsedimentable protein for actin monomers by the inhibition of DNAase I showed that the concentration of monomer was not significantly altered by the presence of villin. Electron microscopic examination of negatively stained, nonsedimentable actin demonstrated that the presence of villin during actin polymerization results in the production of short oligomers which cannot anneal with each other to form long filaments. Villin is also effective in reducing the viscosity of F-actin when it is added to a fuly polymerized actin sample. In view of these striking properties, villin is likely to be an important in vivo regulator of cytoskeletal structure and, by implication, of cell shape and motility.