ELQ-121 is a potent inhibitor of the ubiquinol oxidation (QO) site in parasites. It exhibits an in vitro IC50 of 0.05 nM against both chloroquine-sensitive and multi-drug resistant Plasmodium falciparum. ELQ-121 also inhibits Toxoplasma gondii and Neospora caninum with an in vitro IC50 of less than 1 nM. It hinders the proliferation of Bacteroides fragilis tachyzoites with an IC50 of 0.49 nM and induces mitochondrial disruption. Furthermore, ELQ-121 can be formulated into a polyethylene glycol carbonate prodrug, which shows efficacy against P. yoelii in mice. ELQ-121 is applicable for antimalarial research.