AbstractDuring a REMI screen for proteins regulating cytokinesis in Dictyostelium discoideum we isolated a mutant forming multinucleate cells. The gene affected in this mutant encoded a kinase, SepA, which is an ortholog of Cdc7, a serine‐threonine kinase essential for septum formation in Schizosaccharomyces pombe. Localization of SepA‐GFP in live cells and its presence in isolated centrosomes indicated that SepA, like its upstream regulator Spg1, is associated with centrosomes. Knockout mutants of SepA showed a severe cytokinesis defect and a delay in development. In multinucleate SepA‐null cells nuclear division proceeded normally and synchronously. However, often cleavage furrows were either missing or atypical: they were extremely asymmetric and constriction was impaired. Cortexillin‐I, a marker localizing strictly to the furrow in wild‐type cells, demonstrated that large, crescent‐shaped furrows expanded and persisted long after the spindle regressed and nuclei returned to the interphase state. Outside the furrow the filamentous actin system of the cell cortex showed strong ruffling activity. These data suggest that SepA is involved in the spatial and temporal control system organizing cortical activities in mitotic and postmitotic cells. Cell Motil. Cytoskeleton 66: 929–939, 2009. © 2009 Wiley‐Liss, Inc.