The naphthylisoquinoline alkaloids dioncophylline A (1) and 5'-O-demethyl-8-O-methyl-7-epi-dioncophylline A (2) represent a new structural type of molluscicidal compounds and the active principle of extracts of the tropical lianas Ancistrocladus abbreviates and Triphyophyllum peltatum. The activity of 1 was further improved by derivatization.