After removing lipophilic material, the ground root bark of Quassia africana Baill. (Simaroubaceae) was extracted with ethanol 95 %. Partitioning between chloroform, ethyl acetate and water yielded three crude extracts. Pronounced activities were shown by the chloroform and ethyl acetate crude extracts against Herpes simplex, Semliki forest, Coxsackie and Vesicular stomatitis viruses. By repeated column chromatography and preparative thin layer chromatography on silica gel, two quassinoids, i. e., quassin and simalikalactone D were isolated. Structures of the pure compounds were established primarily using NMR spectroscopy. Mass spectral information confirmed the assigned structures. Simalikalactone D was responsible, at least in part, for the high antiviral activity observed for the chloroform crude extract. Quassin showed no activity. For quassinoids the ester group at C-15 and the epoxymethano bridge between C-8 and C-13 appeared to be important structural features in order to exhibit a pronounced antiviral activity.