Mating in plants is usually mediated by animals, yet few studies have determined whether the mating system of plant populations changes in response to variation in the taxonomic composition or to the foraging behavior of animal pollinators. Here, I investigate covariation between striking geographic variation in pollinator fauna and the mating system of a widespread self‐compatible wetland plant, Decodon verticillatus. Flower visitor surveys in two northern populations from Canada and three southern populations from the southeastern United States revealed that flowers in northern populations were visited primarily by bumblebees (30%), nonnative honeybees (39%), and small, apparently ineffective, pollinators including wasps, flies, and solitary bees (31%). In contrast, southern populations were visited by butterflies (27%), large native bees (Bombus and Xylocopa; 65%), and only rarely by honeybees (2%). The frequency of visits to inflorescences was also much higher in southern ( ) than in northern populations (1.1/h). The foraging behavior of butterflies may enhance outcrossing by reducing geitonogamy, the predominant mode of self‐fertilization in northern populations. Compared to bees, butterflies moved between inflorescences more frequently (64% vs. 43% of the time) and flew longer distances between inflorescences ( vs. 25 cm). Despite large regional differences in visitor composition and movement patterns, multilocus estimates of the proportion of outcrossed progeny did not differ between five southern populations ( ; –0.87) and 10 northern populations ( ; 0.62–0.86). Similarly, the fixation index of mature plants did not differ from 0 in either northern or southern populations. These results indicate that variation in the pollinator fauna, including major geographic shifts in the frequency of butterflies versus bees and native versus nonnative pollinators, does not greatly affect the mating system of D. verticillatus.