SummaryYeasts occur naturally in floral nectar and have been considered to be an influence on the foraging behaviour of bees. When honeybees were presented with a choice of flowers of milkweed,Asclepias syriaca, which were contaminated with yeasts in the nectarvs.yeast-free flowers, they showed no discrimination in visits or movements between the flower types. They moved to the closest flower (reward) regardless of the presence or absence of yeasts. The same results were obtained with the use of yeast-free and yeast-contaminated droplets of sugar syrup.