Two novel bioactive mono-tetrahydrofuran (THF) γ-lactone acetogenins, 16,19-cis-murisolin (1) and murisolin A (2), were isolated from the seeds of Asimina triloba (Annonaceae) by directing the fractionation with brine shrimp lethality. 1 has a 16,19-cis mono-THF ring with two flanking hydroxyl groups and represents a new type among the natural Annonaceous acetogenins. Murisolin (3), which is new in this species, was also obtained. 1-3 showed potent and selective cytotoxicities among six human tumor cell lines.