The mitochondrial outer membrane protein, Mmm1p, is required for normal mitochondrial shape in yeast. To identify new morphology proteins, we isolated mutations incompatible with the mmm1-1 mutant. One of these mutants, mmm2-1, is defective in a novel outer membrane protein. Lack of Mmm2p causes a defect in mitochondrial shape and loss of mitochondrial DNA (mtDNA) nucleoids. Like the Mmm1 protein (Aiken Hobbs, A.E., M. Srinivasan, J.M. McCaffery, and R.E. Jensen. 2001. J. Cell Biol. 152:401-410.), Mmm2p is located in dot-like particles on the mitochondrial surface, many of which are adjacent to mtDNA nucleoids. While some of the Mmm2p-containing spots colocalize with those containing Mmm1p, at least some of Mmm2p is separate from Mmm1p. Moreover, while Mmm2p and Mmm1p both appear to be part of large complexes, we find that Mmm2p and Mmm1p do not stably interact and appear to be members of two different structures. We speculate that Mmm2p and Mmm1p are components of independent machinery, whose dynamic interactions are required to maintain mitochondrial shape and mtDNA structure.