Technical Abstract: The mating system and flower visitors of two threatened species of Sclerocactus (Cactaceae) were studied in the Uintah Basin of eastern Utah, an area undergoing rapid energy development. We found that both S. wetlandicus and S. brevispinus, as well as a third presumptive taxon (undescribed) which we dub S. wetlandicus-var1, cannot produce seeds unless their flowers are visited by pollinators. The two named taxa are predominantly outcrossed and are effectively self-incompatible. The undescribed taxa is fully self-compatible. We found tentative evidence for pollinator limitation of fruit set in S. brevispinus but not in the other taxa. Pollinators are largely species of native ground-nesting bees in the subfamily Halictinae. These bees are indiscriminate collectors of pollen and nectar from Sclerocactus flowers as well as other plant species. The wellbeing of Sclerocactus populations requires that land managers ensure the preservation of halictid bee habitat, which includes both nesting sites and a diversity of floral resources. These bees are also undoubtedly the pollen vectors responsible for the production of hybrids in the zones where S. wetlandicus and S. brevispinus meet. We experimentally confirmed that reciprocal crosses between S. wetlandicus and S. brevispinus are indistinguishable from each other or from pure crosses in fruits/flower, seeds/fruit and seed weights. We suggest that hybridization preceded energy development, that it attests to the ongoing process of evolution in