Apoptotic cell death is a mechanism by which organisms eliminate superfluous or harmful cells. Expression of the cell death regulatory protein REAPER (RPR) in the developing Drosophila eye results in a small eye owing to excess cell death. We show that mutations in thread (th) are dominant enhancers of RPR-induced cell death and that th encodes a protein homologous to baculovirus inhibitors of apoptosis (IAPs), which we call Drosophila IAP1 (DIAP1). Overexpression of DIAP1 or a related protein, DIAP2, in the eye suppresses normally occurring cell death as well as death due to overexpression of rpr or head involution defective. IAP death-preventing activity localizes to the N-terminal baculovirus IAP repeats, a motif found in both viral and cellular proteins associated with death prevention.