The fine regulation of actin polymerization is essential to control cell motility, architecture and to perform essential cellular functions. Formins are key regulators of actin filament assembly, known to processively elongate filament barbed ends and increase their polymerization rate. Different models have been extrapolated to describe the molecular mechanism governing the processive motion of formin FH2 domains at polymerizing barbed ends. Using negative stain electron microscopy, we directly identified for the first time two conformations of the mDia1 formin FH2 domains in interaction with the barbed ends of actin. These conformations agree with the speculated open and closed conformations of the “stair stepping” modelproposed by Otomo and colleagues(Otomo et al., 2005). We observed the FH2 dimers to be in the open conformation for 79% of the data, interacting with the two terminal actin subunits of the barbed end, while they interact with three actin subunits in the closed conformation. In addition, we identified single FH2 dimers encircling the core of actin filaments, providing structural information for mDia1 away from the barbed end, and showing that mDia1 formins are able to explore the core of actin filaments by spontaneously departing from barbed ends.