A great deal is now known about how cells regulate entry into mitosis, but only recently have the mechanisms controlling exit from mitosis and cytokinesis begun to be revealed. In the budding yeast Saccharomyces cerevisiae, Mob1p interacts with the Dbf2p kinase and cells containing mutations in these genes arrest in late anaphase [1] [2]. Proteins related to Mob1p are present in both plants and animals, but information about Mob1p function has been obtained only from budding yeast. Here, we describe the identification and characterization of Mob1p from Schizosaccharomyces pombe. Mob1p associates with the Sid2p kinase and like Sid2p, Mob1p is required for the initiation of cytokinesis, but not for mitotic exit. Mob1p localizes to the spindle pole body (SPB) and to the cell-division site during cell division, suggesting that it might be involved in transducing the signal to initiate cell division from the SPB to the division site. Mob1p is required for Sid2p localization, and Mob1p localization requires the function of the cdc7, cdc11, cdc14, spg1, sid1, sid2, and sid4 genes, suggesting that together with Sid2p, Mob1p functions at the end of the signaling cascade required to regulate the onset of cytokinesis at the end of mitosis.