The infection with gillworms (Dactylogyrus sp.) causes severe problems in the keeping and breeding of tropical ornamental fishes, e.g. Discus (Symphisodon aequifasciata). Treatment of gillworm disease with different drugs has been shown in the past to be insufficient. Therefore, the effectiveness of a long-term use of praziquantel was evaluated in ornamental fish. Cumulative doses up to 10 mg praziquantel/l water were tolerated without side-effects by Angel Fish (Pterophyllum scalare), Discus, and a variety of catfish species (Ancistrus sp., Corydoras sp.). It was found appropriate to start with a dosage of 2.5 mg/l and to add the same dosage every other day several times. All adult parasites and larvae were killed by this treatment. For the complete elimination of the Dactylogyridae-population in a closed aquarium system, 3 therapy-cycles (duration: 5-6 d, accumulated dosage: 2.5 mg/l/d) prooved to be effective. It was important to interrupt the therapy-cycles with intervals without medication (1 to 4 weeks).