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Organ pipe cactus (Stenocereus thurberi) is one of three species of bat-pollinated columnar cacti that co-occur along the Gulf Coastal region of the Sonoran Desert in northwestern Mexico. Its relatively long flowering season ( April-July) overlaps broadly with those of cardon (Pachycereus pringlei) and saguaro (Carnegiea gigantea), but peak flowering occurs ca. 1 mo later ( in June) than the other two species. Some individuals of organ pipe consistently begin flowering early when competition for pollinators with the other species in this pollen-limited cactus is likely to be high. To determine the consequences of early flowering, I conducted hand pollination experiments in which flowers received either conspecific or heterospecific ( cardon) pollen. Fruit set from heterospecific pollination was high ( up to 74%), and ripe fruits contained mature-appearing seeds, apparently produced by some form of agamospermy. These seeds, however, lacked embryos and did not germinate. In April though mid-May, most fruits from open-pollinated flowers at one site in coastal Sonora were produced by heterospecific pollination and contained sterile seeds. Early flowering in such populations should be strongly selected against. Nonetheless, early flowering is adaptive in populations of organ pipe that do not co-occur with cardon because it potentially exposes flowers to large numbers of migrating nectar bats and hummingbirds.