Cytochrome c oxidase from Saccharomyces cerevisiae is composed of nine subunits. Subunits I, II and III are products of mitochondrial genes, while sub-units IV, V, VI, VII, Vila and VIII are products of nuclear genes. To investigate the role of cytochrome c oxidase subunit VII in biogenesis or functioning of the active enzyme complex, a null mutation in the COX7 gene, which encodes subunit VII, was generated, and the resulting cox7 mutant strain was characterized. The strain lacked cytochrome c oxidase activity and haem a/a3 spectra. The strain also lacked subunit VII, which should not be synthesized owing to the nature of the cox7 mutation generated in this strain. The amounts of remaining cytochrome c oxidase sub-units in the cox7 mutant were examined. Accumulation of subunit I, which is the product of the mito-chondrial COX1 gene, was found to be decreased relative to other mitochondrial translation products. Results of pulse-chase analysis of mitochondrial translation products are consistent with either a decreased rate of translation of COX1 mRNA or a very rapid rate of degradation of nascent subunit I. The synthesis, stability or mitochondrial localization of the remaining nuclear-encoded cytochrome c oxidase subunits were not substantially affected by the absence of subunit VII. To investigate whether assembly of any of the remaining cytochrome c oxidase subunits is impaired in the mutant strain, the association of the mitochondrial-encoded subunits I, II and HI with the nuclear-encoded subunit IV was investigated. Relative to the parental COX7+ strain, the cox7 mutant strain showed no association of sub-unit IV with subunit I, and decreased association of subunits IV and II. These results provide direct evidence for a role for subunit VII in assembly between mitochondrial-encoded and nuclear-encoded cytochrome c oxidase subunits.