Abstract: Cationic dyes have been shown to bind efficiently to DNA [1] and lead to photoinduced DNA damage [2]. Such photobiological features may be used to detect or characterize the nucleic acid [3]. Furthermore the intentional photoinduced damage of DNA, e. g. in tumor cells, may be applied in phototherapy [4,5]. Among the cationic aromatic compounds that have been investigated along these lines are coralyne (1) [6] and related quinolizinium salts [7]. During our studies on photobiological properties of aromatic heterocycles we showed that readily available benzo[b]-quinolizinium salts (acridizinium salts) 2 exhibit pronounced DNA-binding and DNA-photodamaging properties [8]. To further investigate the photobiological features of quinolizinium derivatives we synthesized indolo[2,3-b]- quinolizinium salt (3a) [9]. The interactions of this aromatic dye with DNA seemed to be of special interest since it resembles closely the structure of the known anti-tumor compound Céliptium® 4. Herein, we present preliminary investigations of the interactions of quinolizinium dye 3a with DNA.