Abstract The function of villin, an actin-binding protein, has been investigated by transfecting fibroblasts with cloned human cDNAs encoding wild-type villin or functional villin domains. Synthesis of large amounts of villin Induced the growth of numerous long microvilli on cell surfaces together with the redistribution of F-actin. These microvilli contained a cytoskeleton of F-actin, and their appearance was frequently accompanled by the disappearance of stress fibers. The complete villin gene sequence was required to exert its morphogenic effect. Villin lacking one actin-binding domain (113 amino acids), located at its carboxyterminal end, did not induce growth of microvilli or stress fiber disruption. Our results indicate that villin plays a key role in vivo in the morphogenesis of microvilli.