Abstract: The COX5a and COX5b genes encode divergent forms of yeast cytochrome c oxidase subunit V. Although the polypeptide products of the two genes are functionally interchangeable, it is the Va subunit that is normally found in preparations of yeast mitochondria and cytochrome c oxidase. We show here that the predominance of subunit Va stems in part from the differential response of the two genes to the presence of molecular oxygen. Our results indicate that during aerobic growth, COX5a levels were high, while COX5b levels were low. Anaerobically, the pattern was reversed; COX5a levels dropped sevenfold, while those of COX5b were elevated sevenfold. Oxygen appeared to act at the level of transcription through heme, since the addition of heme restored an aerobic pattern of transcription to anaerobically grown cells and the effect of anaerobiosis on COX5 transcription was reproduced in strains containing a mutation in the heme-biosynthetic pathway (hem1). In conjunction with the oxygen-heme response, we determined that the product of the ROX1 gene, a trans-acting regulator of several yeast genes controlled by oxygen, is also involved in COX5 expression. These results, as well as our observation that COX5b expression varied significantly in certain yeast strains, indicate that the COX5 genes undergo a complex pattern of regulation. This regulation, especially the increase in COX5b levels anaerobically, may reflect an attempt to modulate the activity of a key respiratory enzyme in response to varying environmental conditions. The results presented here, as well as those from other laboratories, suggest that the induction or derepression of certain metabolic enzymes during anaerobiosis may be a common and important physiological response in yeast cells.