Field studies were conducted in Indiana, Michigan, Ohio, and Wisconsin in 1992 and 1993 to evaluate growth and response of common ragweed (Ambrosia artemisiifolia) to imazethapyr. Plants from ecotypes originating in each state were grown at all four locations. Nontreated plants grown in Wisconsin or Ohio were consistently taller than plants grown in Michigan or Indiana, regardless of the origin of the seeds. Nontreated plants originating from Wisconsin or Michigan showed a trend toward flowering earlier than those originating from Indiana or Ohio, regardless of test site. The results suggest the existence of common ragweed ecotypes based on origin of the seeds. Weather conditions after treatment with imazethapyr had a greater influence on common ragweed control and regrowth than the origin of common ragweed seeds. Common ragweed regrew following imazethapyr application through the development of axillary buds under conditions of warm temperatures and moist soils during the 4 to 6 wk following imazethapyr application. Mild to cool temperatures or dry conditions following imazethapyr application reduced treated common ragweed regrowth. Both early- and mid-postemergence imazethapyr treatments delayed flowering of all ecotypes. However, flowering of imazethapyr-treated plants followed the same order as nontreated plants, with plants from the Michigan or Wisconsin ecotypes showing a trend toward flowering earlier than those from Indiana or Ohio.