We have isolated the discs overgrown gene of Drosophila and shown that it encodes a homolog of the Casein kinase I(delta)/(epsilon) subfamily and is identical to the double-time gene. However, in contrast to the weak double-time alleles, which appear to affect only the circadian rhythm, discs overgrown alleles, including bona fide null alleles, show strong effects on cell survival and growth control in imaginal discs. Analysis of their phenotypes and molecular lesions suggests that the Discs overgrown protein is a crucial component in the mechanism that links cell survival during proliferation to growth arrest in imaginal discs. This work provides the first analysis in a multicellular organism of Casein kinase I(delta)/(epsilon) functions necessary for survival. Since the amino acid sequences and three-dimensional structures of Casein kinase I(delta)/(epsilon) enzymes are highly conserved, the results suggest that these proteins may also function in controlling cell growth and survival in other organisms.