Gigantetronenin [1] and gigantrionenin [6], two new monotetrahydrofuran Annonaceous acetogenins each possessing a double bond along the hydrocarbon chain, have been isolated from the bark of Goniothalamus giganteus by the use of brine shrimp lethality for bioactivity-directed fractionation. The structures were elucidated based on spectroscopic and chemical methods. Compounds 1 and 6 both show selective and potent cytotoxicities to human tumor cells in culture as well as toxicity to brine shrimp. A known cytotoxic acetogenin, annomontacin [11], was also isolated from this plant. The biogenetic pathway of the acetogenins from G. giganteus is discussed.