The mechanistic details of mtDNA maintenance in petite-negative yeasts have remained largely unexplored. We report here that the DNA helicase Hmi1p plays a crucial role in mtDNA stability in Candida albicans. Like its counterpart in Saccharomyces cerevisiae, Hmi1p in C. albicans (CaHmi1p) contains a C-terminal mitochondrial targeting signal that is functional in both organisms. Biochemical analysis demonstrates that CaHmi1p is a protein possessing ATP-dependent 3'-5' DNA-unwinding activity. Deletion of both HMI1 alleles does not lead to complete loss of mtDNA in C. albicans; however, substantial fragmentation of the wild-type mitochondrial genome, reduction of mtDNA mass and loss of wild-type nucleoid distribution occur. Specific regions of the mitochondrial genome give rise to mtDNA molecule populations with altered characteristics upon CaHMI1 deletion. Fragmentation of the mitochondrial genome can be reversed by reintroduction of CaHmi1p. This is the first time that a gene required for wild-type mtDNA maintenance in S. cerevisiae has been demonstrated to be nonessential in a petite-negative yeast.