Haliotrema abaddon is a significant monogenean parasite of the gills of maricultured Western Australian dhufish, Glaucosoma hebraicum, and can cause severe gill pathology. A series of trials was undertaken using infested fish in 120-l glass aquaria to assess the safety and efficacy of some potential 'in water' treatments, including trichlorphon, praziquantel, formalin, toltrazuril and a freshwater bath. Treatment efficacy was established by assessment of the number of parasites found in tank sediment immediately following treatment and by examination of gills for parasites using a gill biopsy technique. Praziquantel bathes, 2 mg l'1 of 30-h duration, were the most safe and effective treatment. Freshwater bathes of below 2' salinity for 90 min were effective but highly stressful for the fish. Trichlorphon 0.5 mg l'1 for 30 h was safe and relatively effective. Formalin, 25 ppm, and toltrazuril, 12.5 mg l'1, were safe but resulted in the removal of insufficient parasites to recommend it as a treatment for dhufish infested with H. abaddon.