Dioncophylline A (1) represents a novel insecticidal agent, as documented by its enhanced growth-retarding effect on larvae of the polyphagous pest insect Spodoptera littoralis. Within the scope of the work described here, the potential of this as yet most active naphthylisoquinoline alkaloid was further elucidated by the preparation and testing of selected analogues. Investigation of a broad series of structurally modified dioncophylline A analogues (2-20) revealed the free amine function to be essential for the growth inhibitory effect, whereas a modification of the OH function partially led to a distinct increase of activity. In particular, the 8-O-alkyl (especially 8-O-benzylated) derivatives (14 and 15 as well as 16-19) displayed pronounced effects. In the case of 8-O-(p-bromobenzyl)dioncophylline A (16), the activity of the natural parent compound dioncophylline A (1) (EC50 = 277 micrograms/g fresh wt of diet; concentration that inhibits larval growth by 50%) was even improved by a factor of > 15 (EC50 = 15.6 micrograms/g fresh wt).