A composition for controlling microsporidia in fishes is prepared by using, as an active ingredient, at least one kind of compound selected from the group consisting of a compound represented by general formula (I) [wherein R 2 , R 4 , R 5 , R 6 and R 7 independently represent a specific substitute], a pharmaceutically acceptable salt thereof, and a compound capable of forming a compound represented by general formula (I) when metabolized in a fish body. Provided is a method for controlling microsporidia in fishes, said method comprising using the aforesaid composition for controlling microsporidia in fishes so as to exert excellent effects of preventing microsporidia infection in fish muscles or organs, and/or inhibiting the proliferation of microsporidia in fish muscles or organs, and/or exterminating microsporidia from fish bodies, and having high safety.
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