This investigation focuses primarily on those ecological and evolutionary aspects involved in the natural history of the bees that utilize the pollen, nectar, or both, of sunflowers (Helianthus) in North America either for their survival as species or for their maintenance in viable populations. This bee-flower association is examined principally by analyses of the intrafloral, geographic, diurnal, and seasonal interrelationships that exist between this fauna of bees and the various species of Helianthus present in North America. On the basis of this study it is established that more than 400 species of native bees visit the flowers of Helianthus in America north of Mexico. Of these it is those species that have evolved a specialized intrafloral relationship with these flowers that serve as the principal pollinators of these plants.