Formin homology proteins (formins) play critical roles in the formation of actin stress fibers, the yeast actin cable and the cytokinetic contractile ring. Formins bind around the barbed end and processively elongate the actin filament (F-actin). However, it is unclear how and where in the cell formins form these actin-based structures. Here, we show the rotational movement of a mammalian formin mDia1 along the long-pitch helix of F-actin during processive elongation and depolymerization. From these properties, we postulate that helical rotation of formins may regulate actin assembly and filament stabilization during processive F-actin elongation.<br>