Perkinsus marinus is a lethal protozoan that impacts eastern oyster (Crassostrea virginica) populations along the eastern seaboard of the United States and the Gulf of Mexico. Despite the potential advantages of a chemical therapy to effectively treat P. marinus infected oysters for aquaculture and research, no such therapy exists. Given the close taxonomic and phylogenetic proximity of P. marinus to another protozoan parasite, Plasmodium spp., the antimalarial drug quinine was evaluated as a potential chemotherapeutic agent. A concentration of 50 μg/mL (0.13 mM) quinine HCl significantly decreased the viability of six geographic isolates of P. marinus after three hours in vitro. Although quinine HCl effectively decreased P. marinus viability, all concentrations tested decreased oyster hemocyte viability in vitro. Furthermore, concentrations below the effective concentration for P. marinus meronts were either lethal to infected oysters or had no observable effect on parasite infections.