For successful cell division, cytokinesis must be properly timed to occur only after the segregation of chromosomes during mitosis. In the fission yeast Schizosaccharomyces pombe, contractile ring assembly initiates at the onset of mitosis, and ring contraction occurs concomitant with septation at the end of anaphase. Although many of the conserved factors necessary for ring assembly and regulation of cytokinesis have been characterized, still little is known about cell-cycle regulation of events that initiate cytokinesis. The formin cdc12p is an essential ring component with a well-characterized function in F-actin assembly. Here we show that overexpression of a cdc12p fragment bypasses normal cell-cycle controls and induces contractile ring assembly and sometimes even ring contraction and septation, all during interphase. Activation of cytokinesis occurs without the apparent activation of cell-cycle regulators such as polo kinase or the septation initiation network. For this effect, cdc12p contributes at least two separable activities: actin assembly and one or more additional functions in cytokinesis initiation. These observations suggest that the formin cdc12p participates downstream of cell-cycle regulators in a network that drives the initiation of cytokinesis.