Most Annonaceous acetogenins are potently bioactive and offer exciting potential as new antitumor, pesticidal and other bioactivites. The major mode of action of the acetogenin is inhibition of electron transport in the mitochodria. Two known cytotoxic ketolactone Annonaceous acetogenins, (2,4-cis)-isoannonacin (compound 1) and (2,4-trans)-isoannonacin (compound 2), have been isolated from Asimina triloba (Annonaceae) by bioactivity directed fractionation. The structures were characterized on the basis of chemical and spectral data. Compounds 1 and 2, which are known but are first isolated from the seeds of this species, were ob-tained. In brine shrimp lethality test (BST), 1 and 2 exhibited cytotoxicity.