We have identified and isolated a novel yeast nuclear gene (SCO1) which is essential for accumulation of the mitochondrially synthesized subunit II of cytochrome c oxidase (CoxII). Analysis of the mitochondrial translation products in a sco1-1 mutant reveals a strong reduction in CoxII. Examination of mitochondrial transcripts by Northern blot hybridization shows that transcription and transcript maturation of OXI1, the gene coding for CoxII, is not affected. Therefore the SCO1 gene product must be involved in a post-transcriptional step in the synthesis of CoxII. We have isolated a 1.7 kb DNA fragment from a yeast gene bank which carries the functional SCO1 gene. Two RNA species of 0.9 kb and 1.2 kb, respectively, hybridize with this DNA fragment, which is localized on chromosome II. Cells whose chromosomal 1.7 kb fragment has been replaced by the yeast URA3 gene fail to accumulate CoxII and in addition subunit I of cytochrome c oxidase (CoxI). The possibility that the SCO1 gene product is bifunctional, i.e. required for both CoxI and CoxII accumulation, is discussed.