<jats:title>ABSTRACT</jats:title> <jats:p> The in vitro antimalarial activities of 46 alkaloids and extracts from <jats:italic>Strychnos</jats:italic> species were evaluated. Two types of quasidimeric alkaloids exhibit high and selective activities against <jats:italic>Plasmodium</jats:italic> . Strychnopentamine and isostrychnopentamine were active against chloroquine-sensitive and -resistant strains (50% inhibitory concentration [IC <jats:sub>50</jats:sub> ] ≈ 0.15 μM), while dihydrousambarensine exhibited a 30-fold higher activity against the chloroquine-resistant strain (IC <jats:sub>50</jats:sub> = 0.03 μM) than it did against the chloroquine-sensitive strain.