Nuclear gene products replicate and partition mitochondrial DNA (mtDNA) molecules in the yeast Saccharomyces cerevisiae. However, few of the relevant genes have been identified. A mutation that results in temperature-sensitive loss of mtDNA identifies one of these genes, MGM1. Deletion of MGM1 shows that aside from its role in the mitochondrion, the gene has no essential cellular function. The MGM1 protein has a 200-amino-acid region that is highly related to a family of GTP-binding proteins of apparently diverse function that includes the microtubule-binding protein, dynamin D100. The temperature-sensitive strain partitions mtDNA molecules at the restrictive temperature, but a defect in mtDNA synthesis results in a reduction in the number of molecules per cell at each cell division. On the basis of the results of this study, we conclude that cells can partition single mitochondrial genomes, and that when a cell receives a single molecule at division it is able to restore the normal complement of multiple copies.