Intra-population trait dimorphism has attracted considerable attention but has rarely been reported in natural populations. We found that a bimodal floral size distribution was maintained in a population of Lamium album var. barbatum over 4 years. In this population, small and large bee species visited flowers. They tended to visit and pollinate flowers with a floral size matching their size. As a result of this pollinator preference, the fitness of ramets with intermediate floral size was lower than that of long or short floral size. Genetic analysis revealed a subtle genetic differentiation between long or short floral size. Additional genetic analysis showed no evidence of secondary contact with allopatric populations with long or short floral size individuals. These results suggest that, in the population, the bimodal distribution of floral size has a sympatric origin and is maintained by disruptive selection resulting from the pollinator preferences to floral size.