Toxoplasma gondii is an intracellular parasite of the apicomplexan phylum, which consists of many medically important parasitic species. Like most eukaryotic organisms, T. gondii possesses a mitochondrion, an organelle that is important for ATP synthesis. The mitochondrial ATP synthase complex utilises the proton gradient established by the electron transport chain to drive ATP synthesis. This is achieved via the flow of protons through a membrane bound molecular rotor in ATP synthase called F_O, which is coupled to ADP phosphorylation at the F₁ component. The genomes of most apicomplexan parasites, including T. gondii, the causative agent of toxoplasmosis, encode canonical subunits of the F1 domain. However, proteins from the F_O domain are not clearly identifiable. Recent proteomic studies of T. gondii mitochondria have identified novel candidate proteins of ATP synthase in these parasites. One of these proteins, termed T. gondii ATP Synthase Associated Protein-1 (TgASAP-1), is conserved in apicomplexan parasites and closely related organisms. In my project, I demonstrate that TgASAP-1 is an integral membrane protein of the mitochondrial ATP synthase complex of T. gondii. Conditional knockdown of TgASAP-1 impairs parasite growth, resulting in incomplete assembly of the ATP synthase complex and defects in mitochondrial respiration. A conserved arginine residue in TgASAP-1 is important for its function, suggesting that TgASAP-1 may be the previously unidentified subunit a of the F_O rotary motor in T. gondii. My findings highlight the importance of the ATP synthase complex T. gondii mitochondrion and expands work on the novel biology of the parasite’s mitochondrion.