Although the evolutionary role of reproductive isolating mechanisms as causative factors in the origin of species has been questioned (Ehrlich and Raven, 1969), their importance in the maintenance of species distinctness within local plant communities appears to be considerable. Authors have repeatedly stressed the necessity of genetically controlled mechanisms that restrict or prevent gene flow between related plant groups, making it possible for such species or populations to coexist sympatrically and yet remain distinct (e.g., Stebbins, 1974; Levin, 1978). Ir. plant families possessing complex floral structures and entomophilous pollination, mechanical isolation may limit severely the extent of interspecific pollination. However, as Levin (1978) has pointed out, the mere presence of a specialized floral architecture need not imply the presence of strong mechanical barriers. In milkweeds (Asclepias), the mechanisms responsible for maintaining species differences appear particularly well-developed. Despite frequently overlapping distributions of species, only one or two naturally-occurring hybrid combinations are known for the 108 North American species (Woodson, 1954). Pollination is accomplished by nectar-seeking insects which insert specialized pollinia (aggregations of pollen) into narrow chambers within the flower. Because individual insects are known to carry the pollinaria (paired pollinia) of diverse asclepiads, many authors (Grant, 1949; Woodson, 1954; Davis and Heywood, 1963; Macior, 1974; Stebbins, 1974) have suggested that the absence of hybridization results from mechanical isolation, with the specific fit between stigmatic chamber and pollinium acting analogously to a lock and key. Differential positioning of dissimilar pollinarium types on insect pollinators may also limit interspecific transfer of pollen (Macior, 1965). However, the effectiveness of both of the mechanisms proposed above has remained largely untested because extensive examination of flowers for the presence of pollinia of other species has not been made. Furthermore, postpollination barriers to hybridization may exist since attempts to create artificial hybrids in Asclepias have been largely unsuccessful (Moore, 1946; Woodson, 1954). The purpose of the following study was to examine the importance of the proposed mechanical factors for three sympatric species of Asclepias by determining whether interspecific pollination occurred in natural and experimental populations of A. incarnata L., A. verticillata L., and A. syriaca L. Most individuals of all three of the species studied are self-incompatible (Kephart, 1980) and are visited by a wide variety of largely hymenopteran and lepidopteran pollinators (Robertson, 1929; Macior, 1965; Kephart, 1979). Two of the species, A. verticillata and A. incarnata, are considered to be more closely related to each other than either is to A. syriaca (Woodson, 1954). Plants of these species are often found growing in close proximity in disturbed areas of the midwest, yet hybrids between them are unknown.