The interface between apoptosis (programmed cell death) and the cell cycle is essential to preserve homeostasis and genomic integrity. Here, we show that survivin, an inhibitor of apoptosis over-expressed in cancer, physically associates with the cyclin-dependent kinase p34 ^cdc2 on the mitotic apparatus, and is phosphorylated on Thr ³⁴ by p34 ^cdc2 -cyclin B1, in vitro and in vivo . Loss of phosphorylation on Thr ³⁴ resulted in dissociation of a survivin-caspase-9 complex on the mitotic apparatus, and caspase-9-dependent apoptosis of cells traversing mitosis. These data identify survivin as a mitotic substrate of p34 ^cdc2 -cyclin B1 and suggest that survivin phosphorylation on Thr ³⁴ may be required to preserve cell viability at cell division. Manipulation of this pathway may facilitate the elimination of cancer cells at mitosis.