The iridoid glucosides lateroside 1, harpagoside 2, ajugol 3, and aucubin 4 were isolated from an ethanolic extract of the roots of the weed Verbascum thapsus that exhibits antigermination activity on seeds of barley (Hordeum vulgare). Bioassays indicated that at 3 mM concentration, compounds 1, 2, and 4 showed moderate inhibition of seed germination. These compounds also reduced root length when they were assayed on pregerminated seeds at 1 mM to 0.001 mM concentration range. Of all compounds tested, aucubin 4 was the most active against root elongation. Compound 3 showed no activity in the bioassays.