An ethanol extract of Achillea millefolium L. showed repelling properties against the mosquito, Aedes aegypti L. Prepared fractions from the extract contained several active compounds which were characterized by thin layer chromatography, high performance liquid chromatography, gas chromatography and mass spectroscopy. Of 35 compounds tested, the most active were the nitrogen containing compound stachydrine, the carboxylic acids, caffeic, chlorogenic, and salicylic acids, and the phenolic compound pyrocatechol. These substances are earlier reported to occur in A. millefolium with the exception of pyrocatechol. Some further substances with lower activity were characterized for the first time in A. millefolium, i.e., adenine, ferulic and mandelic acid, and the methyl esters of caprylic-, linolenic- and undecylenic acid.