An antiparasitic agent for fish containing an inhibitor of folate synthesis and/or an inhibitor of folate activation as the active substance(s). A combination preparation composed of an inhibitor of folate synthesis and an inhibitor of folate activation is preferable, and a sulfonamide is preferable for the inhibitor of folate synthesis. A dihydrofolate reductase inhibitor, a folate antagonist, etc., can be used as the inhibitor of folate activation. The antiparasitic agent is able to exterminate fish parasites via oral administration. It is particularly effective against parasites belonging to the ciliate group among fish parasites.
