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Premise A central goal of pollination biology is to connect plants with the identity of their pollinator(s). While predictions based on floral syndrome traits are extremely useful, direct observation can reveal further details of a species' pollination biology. The wildflower Phlox drummondii has a floral syndrome consistent with pollination by Lepidoptera. We tested this prediction using empirical data. Methods We observed each step of pollination in P. drummondii. First, we observed 55.5 h of floral visitation across the species range. We used temporal pollinator exclusion to determine the contribution of diurnal and nocturnal pollination to reproductive output. We then quantified P. drummondii pollen transfer by the dominant floral visitor, Battus philenor. Finally, we tested the effect of B. philenor visitation on P. drummondii reproduction by quantifying fruit set following single pollinator visits. Results Battus philenor is the primary pollinator of P. drummondii. Pollination is largely diurnal, and we observed a variety of lepidopteran visitors during the diurnal period. However, B. philenor was the most frequent visitor, representing 88.5% of all observed visits. Our results show that B. philenor is an extremely common visitor and also an effective pollinator by demonstrating that individuals transfer pollen between flowers and that a single visit can elicit fruit set. Conclusions Our data are consistent with the prediction of lepidopteran pollination and further reveal a single butterfly species, B. philenor, as the primary pollinator. Our study demonstrates the importance of empirical pollinator observations, adds to our understanding of pollination mechanics, and offers a specific case study of butterfly pollination.