Bioassay-guided fractionation of the crude methanol extract of Morinda lucida Benth. (Rubiaceae) was carried out by column chromatography and anti-trypanosomal activity was tested using in vitro cultures of Trypanosoma brucei brucei S427. Three active fractions (ML 13-18, ML 19-28 and ML 33-45) were obtained with minimum inhibitory concentration (MIC) values of 6.25, 25 and 12.5 µg/ml, respectively. Fraction ML 33-45 was identified as β-sitosterol/stigmasterol using nuclear magnetic resonance (NMR). β-Sitosterol was evaluated for its cytotoxicity using normal prostate cells (PNT2A) and prostatic carcinoma cell lines (DU-145, PC-3 and LNCAP AS) at a concentration of 100 µM. β-Sitosterol was cytotoxic to all the cell lines. Key words: Morinda lucida, Trypanosoma brucei brucei, beta sitosterol, stigmasterol, cytotoxicity.