Temporal variation in climatic conditions is a regular occurrence for most early spring flowering plants in temperate regions, often producing variation in available pollinators. Evaluating the effectiveness and importance of pollinators to a plant population provides an indication of the relative influence of each visitor on a population's reproductive success and selection on floral traits. This study examined pollinator activity and its effect on seed set in a population of Phlox divaricata L. (Polemoniaceae), investigating variation over the 24-h day and the flowering season for two consecutive years. Eleven species, representing three orders of insects, were documented visiting the study population, although lepidopterans (four species, particularly Hemaris diffinis and Megalographa biloba) accounted for 90% of all visitation. The highest visitation rates (and resulting seed set) occurred during the morning and afternoon diurnal time periods and the evening crepuscular time period. The relative frequencies of the different visitors varied over the 24-h day and between flowering seasons. Most visitors that could be evaluated were found to be effective pollinators. Thus, a pollinator's importance to reproductive success was generally dictated by its visitation frequency, H. diffinis being the most important pollinator. This study highlights the importance of considering temporal variation in effective pollinators in species experiencing variable environmental conditions.