Cytokinesis — the final stage in eukaryotic cell division — is achieved by the constriction of an actomyosin-based contractile ring and its associated plasma membrane, partitioning the cytoplasm into the daughter cells. The process is complete within a few minutes and is tightly regulated in space and time, but the exact mechanisms by which the cell cycle machinery regulates the contractile ring and the microtubules associated with the mitotic apparatus has been unclear. In the March 14 Science, Aaron Straight and colleagues at the Harvard Medical School, Boston, US, reported the discovery of a compound called blebbistatin that holds significant promise as a tool to understand how cytokinesis is regulated (Science, 299:1743-1747, March 14, 2003).