Technical Abstract: We studied the breeding system and pollinators of Nebraska populations of the endangered plant Blowout penstemon, Penstemon haydenii. In contrast to previous reports, this species is one of the few self-incompatible species of Penstemon. Plants cannot reproduce sexually unless flowers are visited by pollinators bearing pollen from another plant; self-pollen is unable to effect fertilization. We found no indication that fruit or seed production was limited by inadequate pollination. Pollinators were several species of native bees which varied from population to population and from year to year; there did not appear to be a reliable Blowout penstemon pollinator fauna. Differences in the pollinator fauna between sites appeared to be due to the dominant vegetation, reeds at one site and upland shrubs at the other. At one site flowers were pollinated mainly by large apid bees in the genera Habropoda and Bombus, and by (mostly) smaller halictid bees. At the second site, pollinators were primarily intermediate sized megachilid bees, particularly in the genus Osmia, and halictid bees. One presumed megachilid Penstemon specialist, Hoplitis pilosifrons, carried few pollen grains and may be more of a parasite than a pollinator. Conservation of this species may require providing nesting habitat for native bees as well as modifying grazing schedules to keep cattle out of areas with plants until seeds are mature and/or dispersed, in addition to relaxation of dune stabilization programs.