Technical Abstract: Even generalist pollinators are typically taxonomic specialists during individual foraging bouts. Such floral constancy restricts pollen flow, and thereby gene flow, between otherwise inter-fertile flowering species, thus serving as an ethological mating barrier. Among incipient species, however, floral traits may have diverged little. In these cases, foraging infidelity by generalist pollinators (honey bees, bumblebees, hummingbirds) is known to facilitate hybridization. Many species of solitary bees are taxonomic specialists for pollen, however. Foragers of such oligolectic bees might be more discriminating connoisseurs when choosing among their limited set of floral hosts. If so, they would contribute to positive assortative mating and host plant speciation. Using randomized mixed arrays of flowers of two sympatric Balsamorhiza species, it was found that female pollen foragers of two species of oligolectic Osmia bees preferred the larger flowered B. macrophylla, but showed no floral constancy whatsoever during individual foraging bouts. In a naturally-spaced array of bouquets, these bees frequently moved fluorescent dye particles (as pollen surrogates) between flowers of these two balsamroot species. Foraging infidelity by these oligolectic bees will contribute to introgression and hybridization where interfertile species of Balsamorhiza meet and flower together.