Treatment of trichodiniasis in recirculation eel farms using formalin (37%) has been found insufficient. We have screened 30 different chemical substances for efficacy against natural infections with trichodinids (Trichodina jadranica) on skin and gills of eels. The following substances showed high parasiticidal effect: acriflavin (25 ppm), bithionol (0.1 ppm), chloramine T (50 ppm), Detarox AP(R) (45 ppm), malachite green (1 ppm), raw garlic (200 ppm), potassium permanganate (20 ppm) and Virkon PF(R) vet. (20 ppm). Preliminary screening revealed that the anthelmintic, bithionol, and the decomposable disinfectants Detarox AP and Virkon PF(R) vet. were potential therapeutics and these were tested on infected eel stocks in recirculation production units. The following treatments showed a high parasiticidal effect: bithionol at 0.1 ppm in a recirculation system including the biofilters; Detarox AP(R) (45 ppm) in recirculation systems with the biofilters temporarily isolated for 1 h and Virkon PF(R) vet. (40 ppm; 25 ppm at start supplemented with 15 ppm 1 h later) in rearing tanks temporarily isolated from recirculation systems for 3 h. (C) 2000 Elsevier Science B.V. All rights reserved.