Subunit VIII of mammalian cytochrome c oxidase (COX; EC 18.104.22.168) exists in at least two isoforms, because different but related polypeptides have been identified in COX isolated from liver and heart of both beef and pig. We have isolated a full length cDNA specifying subunit VIII of human COX from a human liver cDNA library. Sequences hybridizing to this cDNA are present at only one site, the COX8 locus, on human chromosome 11q12-q13. The deduced human polypeptide is 58% identical with COX VIII isolated from beef liver, but only 38% identical with COX VIII isolated from beef heart. Transcriptional analysis shows that an mRNA identical with the isolated cDNA is present in abundant amounts not only in human and monkey liver tissue, but in heart and skeletal muscle as well, tissues not known previously to contain this isoform. Since the only COX VIII subunit found in human heart agrees 100% with the polypeptide deduced from this coxVIII cDNA, it may well be that, in distinction to other mammals, only one form of COX VIII exists in primates.