Human UBIAD1 localizes to mitochondria and converts vitamin K-1 to vitamin K-2. Vitamin K-2 is best known as a cofactor in blood coagulation, but in bacteria it is a membrane-bound electron carrier. Whether vitamin K-2 exerts a similar carrier function in eukaryotic cells is unknown. We identified Drosophila UBIAD1/Heix as a modifier of pink1, a gene mutated in Parkinson's disease that affects mitochondrial function. We found that vitamin K-2 was necessary and sufficient to transfer electrons in Drosophila mitochondria. Heix mutants showed severe mitochondrial defects that were rescued by vitamin K-2, and, similar to ubiquinone, vitamin K-2 transferred electrons in Drosophila mitochondria, resulting in more efficient adenosine triphosphate (ATP) production. Thus, mitochondrial dysfunction was rescued by vitamin K-2 that serves as a mitochondrial electron carrier, helping to maintain normal ATP production.