Two bisabolane-type sesquiterpenes, zingiberene-3,6-beta-endoperoxide (1) and zingiberene-3,6-alpha-endoperoxide (2) have been isolated from the aerial parts of Senecio selloi and Eupatorium rufescens. Their structures were determined by spectroscopic methods. Both compounds show schizonticidal activity against Plasmodium falciparum (EC50 = 10 microg/ml).