Two species of milkweed (Asclepiadaceae) are especially abundant in portions of eastern North America. Common milkweed, Asclepias syriaca L., is most abundant in the northern United States and southeastern Canada, while honeyvine milkweed, Cynanchum laeve (Michaux), is most abundant at central latitudes in the eastern half of the United States. The former is frequently cited as an important host plant for larvae of the monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), while the latter is rarely mentioned in this regard. We compared the performance of monarch larvae on these two plant species. Larvae developed significantly faster on honeyvine milkweed than on common milkweed. Average pupal fresh weights appeared to be slightly greater for individuals reared on common milkweed than those reared on honeyvine milkweed, but the difference was not statistically significant. Similarly, larval survival was about 14% higher on common milkweed, but the difference was not significant. Our results indicate that both common milkweed and honeyvine milkweed are suitable hosts for monarch larvae. Given the abundance of honeyvine milkweed in the east-central United States, this species may be a more important host plant for the monarch than has been generally recognized.