Abstract: Stalled biogenesis of the mitochondrial cytochrome c oxidase (CcO) complex results in degradation of subunits containing redox cofactors. The conserved Oma1 metalloproteinase mediates facile Cox1 degradation in cells lacking the Coa2 assembly factor, but not in a series of other mutants stalled in CcO maturation. Oma1 is activated in coa2Delta cells, but the selective Cox1 degradation does not arise merely from its activation. Oma1 is also active in cells with dysfunctional mitochondria and cox11Delta cells impaired in CcO maturation, but this activation does not result in Oma1-mediated Cox1 degradation. The facile and selective degradation of Cox1 in coa2Delta cells, relative to other CcO assembly mutants, is likely due to impaired hemylation and subsequent misfolding of the subunit. Specific Cox1 proteolysis in coa2Delta cells arises from a combination of Oma1 activation and a susceptible conformation of Cox1.