Diversity of flower traits is often proposed as the outcome of selection exerted by pollinators. Positive directional pollinator-mediated selection on floral size has been widely shown to reduce phenotypic variance. However, the underlying mechanism of maintaining within-population floral color polymorphism is poorly understood. Divergent selection, mediated by different pollinators or by both mutualists and antagonists, may create and maintain such polymorphism, but it has rarely been shown to result from differential behavior of one pollinator. We tested whether different behaviors of the same pollinators in morning and evening are associated with dimorphic floral trait in Linum pubescens, a Mediterranean annual plant that exhibits variable within-population frequencies of dark and light-colored flower tube. Usia bicolor bee-flies, the major pollinators of L. pubescens, are mostly feeding in the flower in the morning, while in the evening they are mostly visiting the flowers for mating. In two years of studying L. pubescens in a single large population in the Carmel, Israel, we found in one year that dark-centered flowers received significantly higher fraction of visits in the morning. Fitness was positively affected by number of visits, but no fitness differences were found between tube-color morphs, suggesting that both morphs have similar pollination success. Using mediation analysis, we found that flower size was under positive directional pollinator-mediated selection in both years, but pollinator behavior did not explain entirely this selection, which was possibly mediated also by other agents, such as florivores or a-biotic stresses. While most pollinator-mediated selection studies show that flower size signals food reward, in L. pubescens it may also signals for mating place, which may drive positive selection. While flower size found to be under pollinator-mediated selection in L. pubescens, but differential behavior of the pollinators in morning and evening did not seem to explain flower color polymorphism.