Provided is a method for exterminating parasites in fish in which a salicylanilide drug is administered to fish. The method for exterminating parasites is effective against parasites belonging to the class Monogenea or the class Trematoda. Oxyclozanide, rafoxanide, and closantel are preferred as the salicylanilide drug. The extermination agent is effective in fish belonging to the order Perciformes, the order Pleuronectiformes, the order Clupeiformes, the order Tetraodontiformes, the order Cypriniformes, the order Anguilliformes, and the order Siluriformes.
