Stem bark of Alstonia boonie (1.24 kg) was exhaustively extracted with 95 % aqueous methanol. The crude extract was defatted with n-hexane and thereafter subjected to extensive chromatography to yield Alstiboonine (1) which was fully elucidated and characterised using 2D NMR analysis. Alstiboonine, crude extract and the n-hexane fraction were subjected to brine shrimps lethality test at varying concentrations. The result showed that all the test samples exhibited significant cytotoxicity.