Preparations from the roots of the Thai medicinal plant, Tiliacora triandra Diels, were found to have antimalarial activity against Plasmodium fakiparum in vitro. Methanol extraction yielded two alkaloidal components (water-soluble and water-insoluble). Only the water-insoluble alkaloidal component showed an increase in antimalarial activity. From this component, five different alkaloids of the bisbenzyl-isoquinoline group were isolated and identified as tiliacorine, tiliacorinine, nor-tiliacorinine A, and two uncharacterized alkaloids G and H. Alkaloid G was found to be the most active in vitro schizontocide (ID50, 344 ng/mL) followed by nor-tiliacorinine A and tiliacorine (ID50, 558 and 675 ng/mL, respectively).