Establishing and maintaining homeostasis is critical to the well-being of an organism and is determined by the balance of cell proliferation and death. Two genes that function together to regulate growth, proliferation, and apoptosis in Drosophila are warts (wts), encoding a serine/threonine kinase, and salvador (sav), encoding a WW domain containing Wts-interacting protein. However, the mechanisms by which sav and wts regulate growth and apoptosis are not well understood. Here, we describe mutations in hippo (hpo), which encodes a protein kinase most related to mammalian Mst1 and Mst2. Like wts and sav, hpo mutations result in increased tissue growth and impaired apoptosis characterized by elevated levels of the cell cycle regulator cyclin E and apoptosis inhibitor DIAP1. Hpo, Sav, and Wts interact physically and functionally, and regulate DIAP1 levels, likely by Hpo-mediated phosphorylation and subsequent degradation. Thus, Hpo links Sav and Wts to a key regulator of apoptosis.