Activity-guided investigation of Cyperus rotundus tubers led to the isolation of patchoulenone, caryophyllene alpha-oxide, 10,12-peroxycalamenene and 4,7-dimethyl-1-tetralone. The antimalarial activities of these compounds are in the range of EC50 10(-4)-10(-6) M, with the novel endoperoxide sesquiterpene, 10,12-peroxycalamenene, exhibiting the strongest effect at EC50 2.33 x 10(-6) M.