Until recently, it has been widely accepted that the stress fibers in cells are really a contractile apparatus that regulate cell motility and contraction. Stress fibers are bundles of actin and myosin filaments that generate an isomeric force in many types of nonmuscle cells. Contraction occurs when the myosin regulatory light chain (MLC) is phosphorylated by MLC kinase (MLCK) in a calmodulin/Ca²⁺-dependent manner. Recent studies have also indicated that Rho-kinase is also involved in regulating nonmuscle cell motility and contractility. Rho-kinase mediates the contraction of stress fibers in a Ca²⁺-independent manner. More rapid and extensive contraction of stress fiber has been observed to be induced by MLCK than by Rho-kinase mediated contraction. When the Rho-kinase activity was inhibited by specific inhibitors, the cells not only lost their stress fibers but they also appeared to lose their cytoplasmic tension. The stress fiber is regulated by two independent kinase systems; namely, one by the Ca²⁺-dependent calmodulin/MLCK-system and the other by the Ca²⁺-independent Rho-kinase-system.