The wild-type functions of the genes ced-3 and ced-4 are required for the initiation of programmed cell deaths in the nematode Caenorhabditis elegans. The reduction or loss of ced-3 or ced-4 function results in a transformation in the fates of cells that normally die; in ced-3 or ced-4 mutants, such cells instead survive and differentiate, adopting fates that in the wild type and associated with other cells. ced-3 and ced-4 mutants appear grossly normal in morphology and behavior, indicating that programmed cell death is not an essential aspect of nematode development. The genes ced-3 and ced-4 define the first known step of a developmental pathway for programmed cell death, suggesting that these genes may be involved in determining which cells die during C. elegans development.