From the ethyl acetate soluble fraction of alcoholic extract of Pteridium aquilinum, two flavonoid pigments, astragalin (kaempferol-3-glucoside) and isoquercitrin (quercetin-3-glucoside), were isolated in the form of the yellow and needle crystals, respectively. Moreover the presence of a small quantity of rutin (quercetin-3-rhamnoglucoside) and two unknown flavonoid pigments was proved by the method of the paper chromatography. For studying the thiamine-decomposing activity of the pigments, 10μg of thiamine was incubated with 10μg of several flavonoid pigments at 70° for one hour at pH 7.0 in a total volume of 10ml and the activity was expressed by the quantity of decomposed thiamine. From the results of this experiment, it was proved that the pigments possessing free orthodihydroxyl group in the side phenyl radical such as quercetin and its glycosides had a marked activity. It is therefore presumed that at least a portion of the so-called "thermostable antithiamine factor" of bracken is isoquercitrin and rutin therein.