The fractionation by solvents of the contents of the seeds ofAnnonamuricata(Annonaceae), guided by the toxic activity towards the brine shrimp larva and the KB cancer cells, has enabled us to isolate two new cytotoxic mono-tetrahydrofuran γ-lactones: corossolone1and corossolin2. The structure of these two cytotoxic acetogenins was determined by analysis of the spectral data, NMR and MS in particular. The chemical correlation between1and2was established by the NaBH4reduction of corossolone.1and2constitute the first examples of mono-tetrahydrofuran acetogenins with an unfunctionalized C-4.Keywords: Annonaceae, acetogenins, cytotoxicity, soursop,Annonamuricata.