Fractionation of the EtOH extract of the bark of Asimina triloba, monitoring by brine shrimp lethality, has led to the isolation and structural elucidation of a novel highly cytotoxic Annonaceous acetogenin, trilobacin [1], in addition to six known compounds: asimicin 2], bullatacin [3], bullatacinone [4], N-p-coumaroyltyramine [5], N-trans-feruloyltyramine [6], and (+)-syringaresinol [7]. Acetogenin 1 was identified as a diastereomer of asimicin [2] by spectral and chemical methods, and both 1 and 2 showed potent and selective cytotoxicities in the NCI human tumor cell line screen.