<jats:title>Abstract</jats:title><jats:p>Nine alkaloids from <jats:italic>Alstonia angustifolia</jats:italic> have been assessed for antiprotozoal activities against <jats:italic>Entamoeba histolytica</jats:italic> and <jats:italic>Plasmodium falciparum in vitro.</jats:italic> Three dimeric alkaloids, macrocarpamine, macralstonine acetate (semisynthetic) and villastonine were found to possess significant activity against both protozoans, but were 4–8 times less potent than emetine against <jats:italic>E. histolytica</jats:italic> and 15–50 times less potent than chloroquine against <jats:italic>P. falciparum.</jats:italic> The monomeric alkaloids, alstonerine, alstophylline, 11‐methoxyakuammicine, norfluorocurarine, pleiocarpamine and vincamajine were all considerably less active than the dimers: macralstonine base was found to be inactive.