Leishmania donovani promastigotes introduced into the bloodstream by sandfly vectors, are exposed to reactive oxygen species like H2O2 during phagocytosis by the host macrophages. H2O2 can induce promastigote death, but the mechanism of induction of this death is not known. Studies presented in this paper demonstrate that exposure to 4 mM H2O2 results in a pattern of promastigote death that shares many features with metazoan apoptosis. Motility and cell survival in these parasites show a gradual decline with increasing doses of H2O2. Features common to metazoan apoptosis, such as nuclear condensation, DNA fragmentation with accompanying DNA ladder formation and loss of cell volume, are observed after exposure to 4 mM H2O2. Within 30 minutes of the exposure, there is a significant increase in the ability of the cell lysates to cleave the fluorogenic tetrapeptide acetyl-Asp-Glu-Val-Asp-7-amino-4-trifluoromethyl coumarin, which is a substrate for the CED-3/CPP32 group of proteases. Pretreatment of cells with a specific inhibitor of CED-3/CPP32 group of proteases, Z-DEVD-FMK, reduces the number of cells showing apoptosis-like features, prevents DNA breakage and inhibits cleavage of a PARP-like protein. Activation of the caspase-like proteases is followed at 2 hours by the cleavage of a poly(ADP)ribose-polymerase-like protein and a reduction in intracellular glutathione concentration. DNA breakdown as detected by TdT labelling of cells and agarose gel electrophoresis is visible at 6 hours. Taken together, the above data show for the first time that there is a distinct pathway for apoptosis-like death in L. donovani.