Many highly mobile species, such as migratory birds, can move and disperse over long distances, yet exhibit high levels of population genetic structuring. Although movement capabilities may enable dispersal, gene flow may be restricted by behavioural constraints such as philopatry. In the present study, we examined patterns of genetic differentiation across the range of a highly mobile, colonial waterbird. American white pelicans (Pelecanus erythrorhynchos) breed across continental North America and are currently experiencing a range expansion, especially on the eastern range limit. To assess patterns of genetic structuring, we sampled 333 individuals from 19 colonies across their North American range. The use of ten variable microsatellite markers revealed high levels of allelic richness with no population differentiation. Both Bayesian and frequentist approaches to examining genetic structuring revealed a single panmictic population. We found no evidence of genetic structuring across the Continental Divide or between migratory and non-migratory colonies. The lack of any genetic structure across the range indicates that, unlike other waterbirds with similar life-history characteristics, extensive gene flow and presumably low philopatry appear to preclude genetic differentiation. The lack of population genetic structure in American white pelicans provides an example of range-wide panmixia, a rare phenomenon in any terrestrial species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102, 583–592.