Further studies on the root bark of Uvaria tonkinensis Finet et Gagnep. (Annonacease) led to the isolation of a new Annonaceous acetogenin, tonkinelin (1). The structure of 1 was elucidated on the basis of spectral data and chemical evidence. This compound represents an unusual type of C37 Annonaceous acetogenin, having neither tetrahydrofuran (THF) nor epoxide rings and possessing only a vicinal diol in the hydrocarbon chain. Preliminary pharmacological tests showed that tonkinelin (1) inhibits human leukemia (HL-60) and human colon adenocarcinoma (HCT-8) cell lines in vitro.