The Caenorhabditis elegans gene ced-9 prevents cells from undergoing programmed cell death and encodes a protein similar to the mammalian cell-death inhibitor Bcl-2 (refs 1-7). We show here that the CED-9 protein is a substrate for the C. elegans cell-death protease CED-3 (refs 8, 9), which is a member of a family of cysteine proteases first defined by CED-3 and human interleukin-1 beta converting enzyme (ICE)(10-12). CED-9 can be cleaved by CED-3 at two sites near its amino terminus, and the presence of at least one of these sites is important for complete protection by CED-9 against cell death. Cleavage of CED-9 by CED-3 generates a carboxy-terminal product that resembles Bcl-2 in sequence and in function. Bcl-2 and the baculovirus protein p35, which inhibits cell death in different species through a mechanism that depends on the presence of its cleavage site for the CED-3/ICE family of proteases(9,13-17), inhibit cell death additively in C. elegans, Our results indicate that CED-9 prevents programmed cell death in C. elegans through two distinct mechanisms : first, CED-9 may, by analogy with p35 (refs 9, 17), directly inhibit the CED-3 protease by an interaction involving the CED-3 cleavage sites in CED-9; second, CED-9 may directly or indirectly inhibit CED-3 by means of a protective mechanism similar to that used by mammalian Bcl-2.