In the present study, methanol extracts of 42 traditional medicinal plants with potent anthelmintic activity against Dactylogyrus intermedius (Monogenea) in goldfish (Carassius auratus) were investigated. Cinnamomum cassia, Lindera aggregata, and Pseudolarix kaempferi exhibited 100% activity and were selected for further evaluation by applying five solvents (petroleum ether, chloroform, ethyl acetate, methanol, and water) for the extraction of the samples, followed by the in vivo bioassay. Among the extracts tested, water and methanol extracts of C. cassia showed the highest efficacies with EC(50) values of 13.2 and 12.3 mg L(-1), showing 100% efficacy against D. intermedius at 30.0 and 40.0 mg L(-1), followed by methanol extract of L. aggregata which demonstrated 100% efficacy at 60.0 mg L(-1) with EC(50) value of 17.1 mg L(-1) after 48 h of exposure. Methanol and ethyl acetate extract of P. kaempferi, which exhibited a 100% efficacy against D. intermedius at 60.0 and 50.0 mg L(-1), revealed similar activity with EC(50) values of 23.5 and 23.3 mg L(-1), respectively. Acute toxicity of these active extracts was investigated on goldfish for 48 h and the corresponding median lethal concentrations (LC(50)) of 56.9, 31.3, 88.7, 168.2, and 165.7 mg L(-1), respectively. These findings indicated that these extracts of the three plants can be developed as preferred natural antiparasitic agents for the treatment of D. intermedius.