Autophagy is an evolutionarily conserved stress response mechanism that often occurs in apoptosis-defective cancer cells and can protect against cell death. In this study, we investigated how apoptosis and autophagy affect each other in cancer cells in response to chemotherapeutic treatment. We found that specific ablation of the proapoptotic function of cytochrome c, a key regulator of mitochondria-mediated apoptosis, enhanced autophagy following chemotherapeutic treatment. Induction of autophagy required Beclin 1 and was associated with blockage of Beclin 1 cleavage by caspase 8 at two sites. To investigate the role of Beclin 1 cleavage in the suppression of autophagy and cell survival, a caspase-resistant mutant of Beclin 1 was knocked into HCT116 colon cancer cells. Beclin 1 mutant knockin resulted in markedly increased autophagy and improved long-term cell survival after chemotherapeutic treatment but without affecting apoptosis and caspase activation. Furthermore, Beclin 1 mutant tumors were significantly less responsive to chemotherapeutic treatment than were wild-type tumors. These results show that chemotherapy-induced apoptosis inhibits autophagy at the execution stage subsequent to cytochrome c release through caspase 8-mediated cleavage of Beclin 1. If apoptosis fails to execute, autophagy is unleashed due to lack of Beclin 1 cleavage by caspases and can contribute to cancer cell survival and therapeutic resistance. Therefore, Beclin 1 may be a useful target for inhibiting autophagy to sensitize chemotherapy.