The mitotic checkpoint ensures proper chromosome segregation by delaying anaphase until chromosomes are aligned on the spindle. Following prolonged spindle damage, however, cells eventually exit mitosis and undergo apoptosis. We show here that a murine homolog of the yeast mitotic checkpoint gene BUB1 localizes to the kinetochore during mitosis. By expressing a dominant- negative mutant, we show that mBub1 is not only required for the checkpoint response to spindle damage, but acts in the timing of a normal mitosis. In addition, when mBub1 function is compromised, cells escape apoptosis and continue cell cycle progression, despite leaving mitosis with a disrupted spindle. These data demonstrate a role for kinetochore-associated mBub1 in regulating exit from mitosis, and suggest functional links between the mitotic checkpoint and subsequent apoptotic events in G1.