Insect exclusion from the flowers of Penstemon caryi showed that fruits are not produced unless pollen is moved between flowers by pollinators. We recorded over 30 species of bees visiting the flowers, and about a third of the bee species were common. Flower visitors were primarily pollen-collecting female bees, most of which also carried pollen on areas of the head that strongly implicated them as pollinators. While larger bees were generally more common on the flowers than smaller bees, there appeared to be no difference between size groups in (1) the percent of females collecting Penstemon pollen, (2) the percent of collected pollen that was Penstemon pollen, or (3) the percent of individuals that carried pollen in areas likely to effect pollination. Two long-tongued species, Anthophora ursina (Apidae) and Osmia brevis (Megachilidae), dominated the collections. These 2 species have very different host associations with Penstemon flowers: the specialist O. brevis collects pollen only from species of beardtongues in the western United States, while the generalist A. ursina uses a wide spectrum of plant species for pollen and seems to express fidelity to Penstemon flowers only when those flowers are abundant. There was no evidence that specialists are superior to generalists as pollinators of P. caryi.