A controlled treatment with mebendazole (Vermox) at 1 mg l^-1 in an intensive eel-culture system showed that the monogenean gill parasites Pseudodactylogyrus anguillae and P. bini from cultured European eel (Anguilla anguilla) were eradicated by the drug. Physico-chemical parameters, the microflora and the microfauna in the fish tanks were only slightly affected by the drug. In-vitro studies revealed different drug-susceptibility of adult P. bini and P. anguillae and some inhibition of parasite egg development. Tolerance tests showed that pigmented eels tolerated concentrations of 1-100 mg l^-1 well for 72 h, whereas a concentration of 500 mg l^-1 was lethal within this period. Glass eels were more susceptible to mebendazole: 20% died after 72 h in 1 mg l^-1, 25-40% died in concentrations between 10 and 100 mg l^-1, and 500 mg l^-1 was lethal within 20 h.