p53 is a representative tumor suppressor whose dysfunction is a major cause of human cancer syndrome. Here we isolated flies lacking Dmp53, which encodes the single Drosophila orthologue of mammalian p53 family. Dmp53 null mutants well developed into adults, only displaying mild defects in longevity and fertility. However, genomic stability and viability of Dmp53 mutants dramatically decreased upon ionizing irradiation. Moreover, mutating Dmp53 abolished irradiation-induced apoptosis and reaper induction. These results indicate that Dmp53 is a central component of DNA damage-dependent apoptotic signaling.