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Aquilegia (Ranunculaceae), a genus of about 100 species of attractive herbs found throughout the Northern Hemisphere, has been studied extensively from a number of points of view. Differentiation within the genus has largely concerned floral characteristics, and several papers have been published concerning these and the presumed phylogeny of the included species (Prazmo, 1965a, 1965b; Taylor, 1967; Taylor and Campbell, 1969). All investi gated species of A quilegia have been diploid (n = 7; Prazmo, 1965b), and all that have been tested have been hybridized. Although no meiotic irregularities have been detected in hybrids, not all are completely interfertile (Prazmo, 1965b) as had been previously assumed (Payson, 1918; Anderson, 1931; Kappert, 1943; Pelton, 1958; Linnert, 1961). Aquilegia has been divided into five major complexes (Grant, 1952; Prazmo, 1965b), based largely on morphology and the results of experimental hybridization. According to Grant, each complex is characterized by a different kind of pollination system and has evolved its distinctive floral characteristics in relation to that system. Of these five complexes two are entirely North American-the canadensis and caerulea complexes. Members of the canadensis complex have red or red and yellow, nodding flowers with short spurs. They generally are considered to be hummingbird pollinated (Schneck, 1901; Graenicher, 1910; Pickens, 1931; Grant, 1952), although Schneck (1901) reported two species of the hawkmoth genus Manduca (Macrosilia) and four genera of bees at the flowers of A. canadensis L. in his garden in Illinois. Aquilegia formosa Fisch., a widespread western columbine, is a member of this group. The caerulea complex is characterized by erect blue, yellow or white flowers with long spurs and petal laminae. Moths and butterflies have been predicted to be the pollinators of this complex (Gray, 1883; Trelease 1883; Payson, 1918; Knuth, 1909). Aquilegia pubescens Coville, a species restricted to high elevations in the Sierra Nevada of California, belongs to the caerulea complex. Prazmo (1965b), Taylor (1967) and Taylor and Campbell (1969) agree that these two complexes are very closely related, and that they have differentiated from a common ancestor in North America. In the Sierra Nevada of California, Aquilegia formosa and A. pubescens are often marginally sympatric, and hybrids are observed rather frequently, if locally. Despite this, they remain largely distinct. Grant (1952) argued that their distinctiveness in this area is maintained largely by differential visitation by pollinating agents, hummingbirds in A. formosa, hawkmoths in A. pubescens. The present study was carried out in an effort to evaluate this hypothesis and to investigate what additional factors might be involved, such as the ecological differences mentioned by Grant (1952) and discussed by Baker (1961).