The nematocidal activity of 38 quassinoids, C19 or C20 compounds isolated from Simaroubaceae, was measured using a species of Diplogastridae (Nematoda) to develop lead parasiticides. Of the various quassinoids tested, samaderines B and E displayed the most potent nematocidal activity with a minimum lethal concentration (MLC) of 2.0 x 10(-5) M. The nematocidal activities of samaderines B and E were 15-fold greater than that of albendazole (3.0 x 10(-4) M), 10-fold greater than that of thiabendazole (2.0 x 10(-4) M) and 7.5-fold greater than that of avermectin (1.5 x 10(-4) M). Thus, samaderines B and E may eventually be used as lead parasiticides. In light of the relationship between the structures of quassinoids and their nematocidal activities, those with potent nematocidal activity may require the elements mentioned. These results should help to further our understanding of nematocidal activity.