The search for antiviral agents against vesicular stomatitis virus, herpes simplex virus type 1 and poliovirus type 2 in plants extracts, led to the isolation of two antipoliovirus flavonoids from the medicinal plant Psiadia dentata (Cass.) DC, Asteraceae: 3-methylkaempferol and 3,4'-dimethylkaempferol. The antipoliovirus activity of both compounds was estimated by comparison with 3-methylquercetin, guanidine and Ro-090179. The most potent inhibitor of poliovirus replication was 3-methylkaempferol, and therefore we investigated its mechanism of action. We showed, using the inhibition of [3H]uridine incorporation in viral RNA and performing a dot-blot with one RNA probe specific for the poliovirus genomic strand RNA, that 3-methylkaempferol inhibits the genomic RNA synthesis of poliovirus.