Fisetin, or 3,3',4',7-tetrahydroxyflavone, is present in fruits and vegetables, and has been previously reported to inhibit the proliferation of a variety of cancer cells. We have demonstrated in a previous work that 20 - 60 µmol/L fisetin inhibits cyclin-dependent kinase activities resulting in cell cycle arrest in HT-29 colon cancer cells. In the present study, we attempted to characterize the mechanisms by which fisetin induces apoptosis in HCT-116 cells. DNA condensations, cleavage of poly (ADP-ribose) polymerase (PARP), and cleavage of caspases 9, 7, and 3 were induced in HCT-116 cells treated with 5 - 20 µmol/L of fisetin. Fisetin induced a reduction in the protein levels of anti-apoptotic Bcl-xL and Bcl-2 and an increase in the levels of pro-apoptotic Bak and Bim. Fisetin did not affect the Bax protein levels, but induced the mitochondrial translocation of this protein. Fisetin also enhanced the permeability of the mitochondrial membrane and induced the release of cytochrome C and Smac/Diablo. Additionally, fisetin caused an increase in the protein levels of cleaved caspase-8, Fas ligand, death receptor 5, and TNF-related apoptosis-inducing ligand, and the caspase-8 inhibitor, Z-IETD-FMK, suppressed fisetin-induced apoptosis and the activation of caspase-3. Furthermore, fisetin increases p53 protein levels, and the inhibition of p53 expression by small interference RNA resulted in a decrease in the fisetin-induced translocation of Bax to the mitochondria, release of mono- and oligo-nucleosome in the cytoplasm, and PARP cleavage. These results show that fisetin induces apoptosis in HCT-116 cells via the activation of the death receptor- and mitochondrial-dependent pathway and subsequent activation of the caspase cascade. The induction of p53 results in the translocation of Bax to the mitochondria, which contributes to fisetin-induced apoptosis in HCT-116 cells.