Activity-directed fractionation of the stem bark of Annona squamosa, monitoring with brine shrimp lethality, led to the isolation of the highly bioactive acetogenins bullatacin [1] and bullatacinone [2], thus demonstrating a new abundant plant source for these potent compounds. A new keto-monotetrahydrofuran acetogenin with a ketolactone terminus, as first seen in bullatacinone [2], was also isolated, characterized by spectral analyses, and named squamone [3]. The cytotoxicities of 3 were increased significantly by reduction of the two keto groups to hydroxyls, and the tetrahydrosquamone [7] and bullatacinone [2] both showed selective cytotoxicities to MCF-7 human breast carcinoma. Liriodenine and (-)-kaur-16-en-19-oic acid were also isolated.