A new polyacetylene was isolated from the roots of Solidago altissima L. by column and thin-layer chromatography. The structure of this polyacetylene was identified as methyl 10-(2-methyl-2-butenoyloxy)-cis-2, cis-8-decadiene-4, 6-diynoate from the results of its spectroscopic and chemical analyses. This polyacetylene, as well as dehydromatricaria ester, inhibited the growth of the seedlings of barnyard millet (Panicum crus-galli L. var. frumeniaceum TRIN.).