Overexpression of mammalian proapoptotic protein Bax induces cell death in plant and yeast cells. Bax Inihibitor-1 (BI-1) gene rescues yeast and plant from Bax-mediated lethality. Using the Arabidopsis BI-1 (AtBI-1) gene controlled under the GAL1 promoter as a cell death suppressor in yeast, Cdf1 (cell growth defect factor-1) was isolated from Arabidopsis cDNA library. Overexpression of Cdf1 caused cell death in yeast, whereas such effect was suppressed by co-expression of AtBI-1. The Cdf1 protein fused with GFP was localized in the mitochondria and resulted in loss of mitochondrial membrane potential in yeast. The Bax-resistant mutant, BRM1, demonstrated tolerance against Cdf1-mediated lethality, whereas Datp4 strain was sensitive to Cdf1. Our results suggest that Cdf1 and Bax cause mitochondria-mediated yeast lethality through partially overlapped pathways.