The mitochondrial electron transport chain (mETC) is a series of membrane embedded enzymatic complexes critical for energy conversion and mitochondrial metabolism. In commonly studied eukaryotes, including humans and animals, complex II, also known as succinate dehydrogenase (SDH), is an essential four-subunit enzyme that acts as an entry point to the mETC, by harvesting electrons from the TCA cycle. Apicomplexa are pathogenic parasites with significant impact on human and animal health. The phylum includes Toxoplasma gondii which can cause fatal infections in immunocompromised people. Most apicomplexans, including Toxoplasma, rely on their mETC for survival, yet SDH remains largely understudied. Previous studies pointed to a divergent apicomplexan SDH with nine subunits proposed for the Toxoplasma complex, compared to four in humans. While two of the nine are homologs of the well-studied SDHA and B, the other seven have no homologs in SDHs of other systems. Moreover, SDHC and D, that anchor SDH to the membrane and participate in substrate bindings, have no homologs in Apicomplexa. Here, we validated five of the seven proposed subunits as bona fide SDH components and demonstrated their importance for SDH assembly and activity. We further find that all five subunits are important for parasite growth, and that disruption of SDH impairs mitochondrial respiration and results in spontaneous initiation of differentiation into bradyzoites. Finally, we provide evidence that the five subunits are membrane bound, consistent with their potential role in membrane anchoring, and we demonstrate that a DY motif in one of them, SDH10, is essential for complex formation and function. Our study confirms the divergent composition of Toxoplasma SDH compared to human, and starts exploring the role of the lineage-specific subunits in SDH function, paving the way for future mechanistic studies. Author summary: Apicomplexans, such as Toxoplasma gondii, are parasites of humans and animals that cause diseases of global importance, such as toxoplasmosis which can be fatal to the immunocompromised. The mitochondrial electron transport chain consists of a series of enzymatic complexes that are needed by the parasite for energy metabolism and for producing important metabolites. The compositions of the chain and its enzymes are different between parasites and their human host. Here, we study the succinate dehydrogenase complex (complex II) of the chain and confirm that the Toxoplasma enzyme has different subunits compared to human and that they are critical for its function. We further start unravelling the role of the parasite subunits within the enzyme's mechanism of action. The mitochondrial electron transport chain is an effective drug target for Toxoplasma and other related pathogens such as the malaria causing parasite. We show here that the complex II is important for parasite growth, and validate its differences from the human version, thus enhancing our understanding of this critical parasite pathway. [ABSTRACT FROM AUTHOR]