lnstitute of Systems Biology and Ecology AS CR, Na Sadkach 7, CZ-37005 Ceske Budejovice, Czech Republic E-mail: nada@usbe.cas.cz University of South Bohemia, Faculty of Agriculture, Studentska 13, CZ-37005 Ceske Budejovice. Czech Republic Manuscript received 24 December 2009, accepted 28 April 2010 This study was designed to study the phenolic contents of above-ground parts of all three knotweed species, which have not been investigated yet. Phenolic compounds in methanolic extract of knotweed species (Polygonum cuspidatum Siebold and Zucc., P. sachalinense (F. Schmidt) and P. x bohemicum (Chrtek and Chrtkova) Zika and Jacobson, Polygonaceae) were investigated using high performance liquid chromatography (HPLC) and capillary electrophoresis (CE). Dried samples of flowers and stems were used. We determined catechins, piceid and free quercetin and a large amount of total quercetin In the flowers, and epicatechin, chlorogenic acid, resveratrol, piceid and resveratrol derivative in the stems of all investigated species. Differences in contents of found compounds among species and between parts of plant bodies were ascertained (P < 0.001). The more content or total quercetin was determined in the flowers than in the stems. The results showed that the stems are rich with piceid and chlorogenic acid in P. cuspidatum (about 238 and 103 mg/kg of dry mass, respectively) and the flowers with quercetin-3-β-D-glucoside in P. sachalinense (about 5709 mg/kg of dry mass).