In addition to the primary rewards of pollen and nectar, flowering plants attract pollinators via a suite of visual, olfactory, structural, and/or tactile cues. One of these, blue fluorescence is often displayed by anthers and pollen grains under UV irradiation, and has been suggested as a visual cue for pollinating bees. In this study, the author isolated the major blue fluorescent compound responsible for the UV-induced fluorescence exhibited by the anthers and pollen of the mellitophilous plant species, the Welsh onion (Allium fistulosum L.), and identified it as l-tryptophan, one of essential amino acids for bees. The emission spectrum of l-tryptophan has its maximum at λ_Em 350 nm (UV region) extending beyond 400 nm (blue region). Its fluorescence appears blue to humans who can sense its emission only above 400 nm; but bees, whose visual range is 300–650 nm, perceive it as UV. In a two-choice behavioral assay, the fiery-tailed bumblebee (Bombus ignitus Smith) showed a statistically significant preference for a sucrose solution supplemented with l-tryptophan over non-supplemented one. The l-tryptophan in pollen may act as a visual cue in addition to being a nutrient.