Polymorphic forms of Compound (1) or a pharmaceutically acceptable salt thereof, wherein Compound (1) is represented by the following structural formula:are Form A of HCl salt of Compound(1)·1/2H<；sub>；2<；/sub>；O, Form F of HCl salt of Compound(1)·3H<；sub>；2<；/sub>；O, Form D of HCl salt of Compound(1),Form A of Compound(1),and Form A of tosylate salt of Compound(1).Such polymorphic forms are employed for treating influenza, inhibiting the replication of influenza viruses, or reducing the amount of influenza viruses in a biological sample or in a subject.
