Cluster tarweed (Madia glomerata Hook.) is an annual which occupies rangeland from Saskatchewan to N. Arizona at medium to high elevations. In the laboratory dried tarweed plants and leachates from them reduced the germination of intermediate wheatgrass (Agropyron intermedium (Host) Beauv.) and radish, and caused high proportions of abnormal seedlings. The inhibitory agent(s) was highly water soluble, dialyzable, stable to boiling, drying, and prolonged storage in solution, destroyed by ashing, and not affected by pH 4 to 7. In soil in the greenhouse, tarweed leachates and dried tarweed significantly reduced growth of wheatgrass seedlings. In the field, tarweed infestation reduced the establishment and growth of wheatgrass sown in it in proportion to the stand of tarweed.