Oxygen is crucial for the survival of many organisms, as it functions as the final electron acceptor in aerobic respiration and as a substrate in essential biosynthetic reactions. Several cell types experience hypoxia, or reduced oxygen levels, including human cells within tumors and tissues during cardiovascular disease. Cells of the yeast Saccharomyces cerevisiae are similar to human cells in that they adapt to hypoxia by mounting a large gene expression response. One of the mediators of this response is the heme-dependent transcription factor Hap1, functionally analogous to human Rev-Erb. Hap1 activity is regulated directly by binding the biomolecule heme, which converts Hap1 from a repressor to an activator of genes involved in aerobic respiration and other cellular processes. Heme biosynthesis requires oxygen, so the level of cellular heme is dependent upon oxygen level. Despite Hap1 playing a critical role in regulating cellular respiration, hap1Δ cells do not have an obvious phenotype during aerobic or hypoxic conditions. Thus, we took several approaches to test how Hap1 contributes to cellular function and fitness. First, we found that deleting the HAP1 gene had no effect on replicative lifespan, showing that dysregulation of respiration does not affect aging. Second, we found through competitive growth assays that a strain without HAP1 exhibited a lower fitness per generation compared to a wild type strain. This lower fitness was observed on both fermentable and non-fermentable carbon sources, demonstrating that Hap1 is required for optimal cellular fitness by promoting aerobic respiration. Third, we wanted to test whether Hap1 is important in fermentation as well, so we constructed a strain unable to undergo aerobic respiration. In this strain, deleting HAP1 does not affect growth on glucose, indicating that Hap1 does not play a role in fermentative respiration. Together, our results suggest that while Hap1 is not absolutely required for respiration, Hap1 provides a fitness advantage by fine-tuning aerobic respiration.