Interest in the population dynamics and geographic distribution of common milkweed (Asclepias syriaca L.) has recently increased due to the importance of common milkweed in the life cycle of the monarch butterfly (Danaus plexippus). A survey of common milkweed occurrence in various habitats was conducted in Iowa in June and July of 1999. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn (Zea mays L.) and soybean (Glycine max L. Merr.) fields. Corn and soybean fields had 85% fewer patches than roadsides. Conservation reserve program fields had the greatest average area infested. While common milkweed was frequently found in corn and soybean fields, average frequency and patch sizes were much greater in noncrop areas.