Aims: To develop alternative approaches for medical and environmental control of pathogenic Acanthamoeba spp. by means of photodynamic treatment with a tetracationic Zn(II)-phthalocyanine (RLP068). Methods and Results: Incubation of cyst cultures with RLP068 for 1 h caused an accumulation of readily detectable concentrations of the phthalocyanine, even at doses as low as 0.5 mu mol l(-1). RLP068 exhibited no dark toxicity towards cysts up to 5 mu mol l(-1) concentration. A decrease of c. 50% in cyst survival in comparison with controls was measured upon incubation of the cysts with 0.5 mu mol l(-1) RLP068, followed by exposure to light (600-700 nm) for 20 min at a fluence rate of 50 mW cm(-2) (60 J cm(-2)). After incubation with 3 and 5 mu mol l(-1) RLP068 and irradiation, the cysts lost their excystment ability as early as day 5 and up to day 10, and were clearly damaged when observed under an interference contrast microscope. Conclusions: These data indicate the promising use of RLP068 in phototreatment of diseases caused by pathogenic amoebae and in initial disinfection of wastewaters. Significance and Impact of the Study: Rapid and extensive photodamage may be induced in the highly resistant cystic stages by means of 600-to 700-nm light sources.