In research for successful treatment of fish parasitized by monogeneans, four substances (Praziquantel, Niclosamide, Levamisole-HCl, and Metrifonate) were tested in vivo against Gyrodactylus aculeati parasitizing the skin of sticklebacks (Gasterosteus aculeatus). Fish were incubated in water containing different solutions of the drugs. Praziquantel, Niclosamide and Levamisole-HCl were effective against Gyrodactylus aculeati. Praziquantel caused irreversible lesions in the parasite tegument (beginning with 1 microgram/ml and 90 min exposure). Niclosamide was effective in a narrow concentration range of 0.075-0.1 microgram/ml (90 min). Levamisole-HCl was effective in a concentration range of 20-50 micrograms/ml for 120 min. Like Praziquantel, Niclosamide and Levamisole-HCl led to damage of the parasite tegument. The opisthaptor region and the openings of the cephalic glands were most severely affected. Furthermore, the effects of Niclosamide, Levamisole-HCl and Metrifonate on Diplozoon paradoxum parasitizing the gills of chubs (Squalius cephalus) and breams (Abramis brama) were investigated. Levamisole-HCl (10 micrograms/ml) and Niclosamide (0.2 microgram/ml) were effective against Diplozoon paradoxum after an exposure of 90 min and 45 min in vitro. The parasites were severely affected along the midbody. Metrifonate caused a lysis of the tegument and a strong secretion of slime. It is suggested that chemotherapy against Gyrodactylus sp. may be accomplished with 10 mg Praziquantel/l for 3 h during storage in smaller tanks. Niclosamide (0.1 mg/l for 90 min), or Levamisole-HCl (50 mg/l for 120 min) may be used alternatively. Concentrations of Niclosamide and Levamisole-HCl have to be calculated with accuracy, since fish only tolerate a very narrow range of these drugs (Niclosamide: 0.1 mg/l for 120 min; Levamisole-HCl: 50 mg/l for 120 min).