The isolation and structure elucidation of isoancistrocladine, a novel naphthylisoquinoline alkaloid from the roots of Ancistrocladus heyneanus, is described. The structural attribution was greatly facilitated by the fact that isoancistrocladine had already been prepared before, by reduction of the natural product ancistrocladinine. Isoancistrocladine is the first naturally occurring naphthylisoquinoline alkaloid that has a relative cis-configuration at C-1 versus C-3 without being methylated on the nitrogen. © 1994.