Gyrodactylids are commonly occurring fish ectoparasites that cause high host mortality and considerable economic losses in aquaculture. With the goal of finding safe and effective alternative agents to control monogenean infections in aquaculture, we investigated the anthelmintic efficacy of three common disinfectants (trichloroisocyanuric acid, glutaraldehyde and potassium permanganate) and crude extracts of four herbal medicines (Cortex meliae, Radix zanthoxyli, Semen aesculi and Macleaya cordata) against Gyrodactylus kobayashii in goldfish. Also, we compared the anthelmintic efficacy of the above agents with formaldehyde, which is one of the most frequently-used agents to treat gyrodactyliasis. After 48-h exposure, trichloroisocyanuric acid had the EC50 value of 0.12 mg/L and EC90 value of 0.65 mg/L, and exhibited a 100% efficacy against G. kobayashii at 1.0 mg/L. Glutaraldehyde and potassium permanganate were less effective, with EC50 values of 4.0 and 2.9 mg/L, and EC90 of 5.0 and 3.6 mg/L, respectively. For the extracts of herbal medicines, EC50 and EC90 values of methanol extract of M. cordata were 8.6 and 25.5mg/L, respectively, after 48-h exposure, and EC100 of ethyl acetate and petroleum ether extracts of M. cordata were 80 and 100 mg/L, respectively. The remaining extracts were either highly toxic to fish, or exhibited very weak anthelmintic efficacy, even at higher concentrations. Acute toxicity assay indicated that the LC50 of the promising agents (trichloroisocyanuric acid, glutaraldehyde, potassium permanganate and methanol extract of M. cordata) against goldfish after 48-h exposure were 4.1, 8.8, 5.9 and 81.4 mg/L, which were 6.3, 1.8, 1.6 and 3.2 times higher than the corresponding EC90. The LC50/EC90 ratio for formaldehyde (2.1-fold) was higher than for glutaraldehyde and potassium permanganate, but lower than for trichloroisocyanuric acid and methanol extract of M. cordata. These results suggested that trichloroisocyanuric acid and methanol extract of M. cordata are both viable alternatives for formaldehyde in aquaculture. Statement of relevance: The objective of the present study was motivated by the urgent need to find safe and effective alternative agents to control monogenean infections in aquaculture. (C) 2016 Elsevier B.V. All rights reserved.