An attempt was made to verify the existence of 18 disjunct populations of river birch (Betula nigra L.) displayed on the most current range-wide maps of the species. Communications with numerous botanists and resource managers led to the conclusion that only 4 of the 18 disjunct populations currently exist, and that the bulk of the remaining populations are artifacts arising from horticultural introductions and misidentification. Such overestimates of the occurrence of disjunct populations in widespread tree species of North America is probably common given the extreme difficulties in defining distributional limits.